Update rails head build to reference main instead of master

[HParker]

Rails default branch is now main the build should reflect that

adding the branch prevents the error,

Git error: command `git rev-parse --verify master` in directory /Users/hparker/.rbenv/versions/3.0.0/lib/ruby/gems/3.0.0/cache/bundler/git/rails-fcf0202857b07db1a0f6220dae5ca99319ca0f32 has failed.
Revision master does not exist in the repository https://github.com/rails/rails.git. Maybe you misspelled it?
If this error persists you could try removing the cache directory '/Users/hparker/.rbenv/versions/3.0.0/lib/ruby/gems/3.0.0/cache/bundler/git/rails-fcf0202857b07db1a0f6220dae5ca99319ca0f32'

which can happen your first time running this build.