[Gemfile] Specify Ruby version using `file` argument, attempt 2

[garyhtou]

Reverts hackclub/hcb#6158

Here is the build error:

-----> Build succeeded!
-----> Ruby app detected
-----> Installing bundler 2.5.6
-----> Removing BUNDLED WITH version in the Gemfile.lock
-----> Compiling Ruby/Rails
-----> Using Ruby version: ruby-3.1.5
-----> Installing dependencies using bundler 2.5.6
       Running: BUNDLE_WITHOUT='development:test' BUNDLE_PATH=vendor/bundle BUNDLE_BIN=vendor/bundle/bin BUNDLE_DEPLOYMENT=1 bundle install -j4
       Bundle complete! 121 Gemfile dependencies, 261 gems now installed.
       Gems in the groups 'development' and 'test' were not installed.
       Bundled gems are installed into `./vendor/bundle`
       Bundle completed (0.50s)
       Cleaning up the bundler cache.
       Removing bundler (2.5.6)
-----> Detecting rake tasks

 !
 !     Could not detect rake tasks
 !     ensure you can run `$ bundle exec rake -P` against your app
 !     and using the production group of your Gemfile.
 !     /tmp/build_a14f6c82/vendor/ruby-3.1.5/lib/ruby/3.1.0/bundler/definition.rb:432:in `validate_ruby!': Your Ruby version is 3.1.5, but your Gemfile specified  (Bundler::RubyVersionMismatch)
 !     from /tmp/build_a14f6c82/vendor/ruby-3.1.5/lib/ruby/3.1.0/bundler/definition.rb:407:in `validate_runtime!'
 !     from /tmp/build_a14f6c82/vendor/ruby-3.1.5/lib/ruby/3.1.0/bundler.rb:155:in `setup'
 !     from /tmp/build_a14f6c82/vendor/ruby-3.1.5/lib/ruby/3.1.0/bundler/setup.rb:20:in `block in <top (required)>'
 !     from /tmp/build_a14f6c82/vendor/ruby-3.1.5/lib/ruby/3.1.0/bundler/ui/shell.rb:136:in `with_level'
 !     from /tmp/build_a14f6c82/vendor/ruby-3.1.5/lib/ruby/3.1.0/bundler/ui/shell.rb:88:in `silence'
 !     from /tmp/build_a14f6c82/vendor/ruby-3.1.5/lib/ruby/3.1.0/bundler/setup.rb:20:in `<top (required)>'
 !     from <internal:/tmp/build_a14f6c82/vendor/ruby-3.1.5/lib/ruby/3.1.0/rubygems/core_ext/kernel_require.rb>:85:in `require'
 !     from <internal:/tmp/build_a14f6c82/vendor/ruby-3.1.5/lib/ruby/3.1.0/rubygems/core_ext/kernel_require.rb>:85:in `require'
 !     from /tmp/build_a14f6c82/config/boot.rb:5:in `<top (required)>'
 !     from /tmp/build_a14f6c82/bin/rake:4:in `require_relative'
 !     from /tmp/build_a14f6c82/bin/rake:4:in `<main>'
 !
/tmp/codon/tmp/buildpacks/50d5eddf222a9b7326028041d4e6509f915ccf2c/lib/language_pack/helpers/rake_runner.rb:100:in `load_rake_tasks!': Could not detect rake tasks (LanguagePack::Helpers::RakeRunner::CannotLoadRakefileError)
ensure you can run `$ bundle exec rake -P` against your app
and using the production group of your Gemfile.
/tmp/build_a14f6c82/vendor/ruby-3.1.5/lib/ruby/3.1.0/bundler/definition.rb:432:in `validate_ruby!': Your Ruby version is 3.1.5, but your Gemfile specified  (Bundler::RubyVersionMismatch)
	from /tmp/build_a14f6c82/vendor/ruby-3.1.5/lib/ruby/3.1.0/bundler/definition.rb:407:in `validate_runtime!'
	from /tmp/build_a14f6c82/vendor/ruby-3.1.5/lib/ruby/3.1.0/bundler.rb:155:in `setup'
	from /tmp/build_a14f6c82/vendor/ruby-3.1.5/lib/ruby/3.1.0/bundler/setup.rb:20:in `block in <top (required)>'
	from /tmp/build_a14f6c82/vendor/ruby-3.1.5/lib/ruby/3.1.0/bundler/ui/shell.rb:136:in `with_level'
	from /tmp/build_a14f6c82/vendor/ruby-3.1.5/lib/ruby/3.1.0/bundler/ui/shell.rb:88:in `silence'
	from /tmp/build_a14f6c82/vendor/ruby-3.1.5/lib/ruby/3.1.0/bundler/setup.rb:20:in `<top (required)>'
	from <internal:/tmp/build_a14f6c82/vendor/ruby-3.1.5/lib/ruby/3.1.0/rubygems/core_ext/kernel_require.rb>:85:in `require'
	from <internal:/tmp/build_a14f6c82/vendor/ruby-3.1.5/lib/ruby/3.1.0/rubygems/core_ext/kernel_require.rb>:85:in `require'
	from /tmp/build_a14f6c82/config/boot.rb:5:in `<top (required)>'
	from /tmp/build_a14f6c82/bin/rake:4:in `require_relative'
	from /tmp/build_a14f6c82/bin/rake:4:in `<main>'
	from /tmp/codon/tmp/buildpacks/50d5eddf222a9b7326028041d4e6509f915ccf2c/lib/language_pack/ruby.rb:956:in `rake'
	from /tmp/codon/tmp/buildpacks/50d5eddf222a9b7326028041d4e6509f915ccf2c/lib/language_pack/rails4.rb:69:in `block in run_assets_precompile_rake_task'
	from /tmp/codon/tmp/buildpacks/50d5eddf222a9b7326028041d4e6509f915ccf2c/lib/language_pack/base.rb:192:in `log'
	from /tmp/codon/tmp/buildpacks/50d5eddf222a9b7326028041d4e6509f915ccf2c/lib/language_pack/rails4.rb:63:in `run_assets_precompile_rake_task'
	from /tmp/codon/tmp/buildpacks/50d5eddf222a9b7326028041d4e6509f915ccf2c/lib/language_pack/ruby.rb:100:in `block in compile'
	from /tmp/codon/tmp/buildpacks/50d5eddf222a9b7326028041d4e6509f915ccf2c/lib/language_pack/ruby.rb:977:in `allow_git'
	from /tmp/codon/tmp/buildpacks/50d5eddf222a9b7326028041d4e6509f915ccf2c/lib/language_pack/ruby.rb:93:in `compile'
	from /tmp/codon/tmp/buildpacks/50d5eddf222a9b7326028041d4e6509f915ccf2c/lib/language_pack/rails2.rb:55:in `compile'
	from /tmp/codon/tmp/buildpacks/50d5eddf222a9b7326028041d4e6509f915ccf2c/lib/language_pack/rails3.rb:37:in `compile'
	from /tmp/codon/tmp/buildpacks/50d5eddf222a9b7326028041d4e6509f915ccf2c/lib/language_pack/rails4.rb:30:in `compile'
	from /tmp/codon/tmp/buildpacks/50d5eddf222a9b7326028041d4e6509f915ccf2c/lib/language_pack/rails6.rb:17:in `compile'
	from /tmp/codon/tmp/buildpacks/50d5eddf222a9b7326028041d4e6509f915ccf2c/bin/support/ruby_compile:19:in `block in <main>'
	from /tmp/codon/tmp/buildpacks/50d5eddf222a9b7326028041d4e6509f915ccf2c/lib/language_pack/base.rb:192:in `log'
	from /tmp/codon/tmp/buildpacks/50d5eddf222a9b7326028041d4e6509f915ccf2c/bin/support/ruby_compile:18:in `<main>'
 !     Push rejected, failed to compile Ruby app.
 !     Push failed