Avoid redundant Caskroom chgrp

[MikeMcQuaid]

• Check the Caskroom group before running chgrp.
• Share the expected group name between checks and fixes.
• Keep Linux using sudo when the group must change.

Fixes #22120

---

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them? Performance claims (e.g. "this is faster") must include Hyperfine benchmarks.
• Have you written new tests (excluding integration tests) for your changes? Here's an example.
• Have you successfully run brew lgtm (style, typechecking and tests) with your changes locally?

---

• AI was used to generate or assist with generating this PR. Please specify below how you used AI to help you, and what steps you have taken to manually verify the changes. Non-maintainers may only have one AI-assisted/generated PR open at a time.

Used OpenAI Codex with substantial local refactoring.

---

[Copilot]

Pull request overview

This PR reduces unnecessary sudo prompts during Cask installs by skipping redundant chgrp on the Caskroom when its group is already correct, and by centralizing the expected group name (macOS: admin, Linux: root).

Changes:

• Skip chgrp in Cask::Caskroom.ensure_caskroom_exists when the directory already has the expected group.
• Introduce expected_caskroom_group and use it for both group checks and chgrp arguments.
• Add unit specs covering the new group-check behavior.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
Library/Homebrew/cask/caskroom.rb	Adds group-correctness check and shared expected group helper to avoid redundant chgrp.
Library/Homebrew/extend/os/linux/cask/caskroom.rb	Aligns Linux chgrp implementation with shared expected group helper (still uses sudo).
Library/Homebrew/test/cask/caskroom_spec.rb	Adds specs for skipping/performing chgrp and for expected-group checks on macOS/Linux.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.