fix: allow read-only global list with inaccessible locks - #1975

Summary

• Keep the existing per-venv lock for pipx list when it can be acquired.
• When a lock file is inaccessible, continue listing the readable venv instead of failing a read-only command.
• Add a regression test and changelog fragment for pipx list --global fails for non-root users due to .lock file permissions in global venvs #1974.

Why

Global venvs installed by root can leave lock files that non-root users cannot open. pipx list --global then fails before it reads any installed environment.

Validation

• Targeted Ruff check and format check passed.
• Verified the read-only fallback with a mocked inaccessible lock, including successful list rendering.
• Verified the normal lock acquisition and release lifecycle.
• git diff --check passed.
• The full pytest suite did not complete locally before test initialization timed out; upstream CI will exercise the full suite and Linux permissions behavior.

Checklist

• Ran pre-commit run --all-files (not run locally; targeted Ruff checks passed).
• Wrote descriptive pull request text.
• Added a regression test.
• Added changelog.d/1974.bugfix.md.
• Documentation update is not needed for this read-only failure path.