Missing `Gemfile.lock` error includes `ls: cannot access 'la': No such file or directory`

[edmorley]

STR:

1. mkdir testcase && cd $_
2. touch Gemfile
3. pack build --buildpack heroku/ruby testcase

Expected

• Error message that explains Gemfile.lock is missing, and for bonus points includes the directory listing of the source.
• For that error message not to include any additional errors or warnings generated whilst rendering the error message.

Actual

===> BUILDING

## Heroku Ruby Buildpack

- Could not find `/workspace/Gemfile.lock`, details:
  - failed to open file `/workspace/Gemfile.lock`: No such file or directory (os error 2)
- Debug info Contents of the `/workspace` directory
  - Running debug command `ls la /workspace`

      ls: cannot access 'la': No such file or directory
      /workspace:
      Gemfile
      README.md
      bin

  - Done (< 0.1s)
  - Command failed `ls la /workspace`
    exit status: 2
    stdout: <see above>
    stderr: <see above>

! Error: `Gemfile.lock` not found
!
! A `Gemfile.lock` file is required and was not found in the root of your application.
!
! If you have a `Gemfile.lock` in your application, ensure it is tracked in Git and
! that you’re pushing the correct branch.
!
! For more information:
! https://devcenter.heroku.com/articles/git#deploy-from-a-branch-besides-main

Note the ls: cannot access 'la': No such file or directory.

Suggested next steps:

1. Fix the args to ls (to use -la instead of la)
2. Add libcnb-test integration test coverage of the full output here (such a test would be fast to run, since it's a failure right at the start of the build, plus IMO the error messages around onboarding an app to the CNB are high priority for UX/DX and so worth ensuring all are tested)
3. Ideally reducing some of the unnecessary repetition in the build output. For example there's a lot of duplication around Could not find + No such file or directory (os error 2) before the actual error message. For some other type of I/O error, there is value in exposing the underlying error, but for this "file not found" case I don't think that pre-amble is needed (the only useful part of it is the absolute path to the Gemfile.lock, which could be inlined in the actual error message etc).

cc @schneems