Summary

Shell builtins (cd, echo, pwd, export, etc.) don't exist as files in the PATH - they're built into the shell itself. Previously, pipenv run cd would fail with:

Error: the command cd could not be found within PATH or Pipfile's [scripts].

Changes

Modifies do_run_posix in pipenv/routines/shell.py to fall back to running commands through the shell when they're not found in PATH, similar to the Windows behavior that already existed in _launch_windows_subprocess.

Before (POSIX)

1. Try to find command in PATH using system_which
2. If not found → error and exit

After (POSIX)

1. Try to find command in PATH using system_which
2. If found → use os.execve for direct execution (unchanged)
3. If not found → fall back to subprocess.run with shell=True (allows shell builtins to work)

Testing

• Added integration test test_run_shell_builtins that tests echo, pwd, and cd builtins
• Manual testing confirms that pipenv run echo, pipenv run pwd, and pipenv run cd now work correctly

Fixes #6186

Pull Request opened by Augment Code with guidance from the PR author