Keep generic tool caches out of the sandboxed test home

[p-linnane]

brew tests points HOME at Library/Homebrew/test to sandbox local configuration, but leaves XDG_CACHE_HOME unset. Tools that honor the XDG defaults, e.g. RuboCop, then resolve their cache to $HOME/.cache and leave a stray Library/Homebrew/test/.cache directory inside the working tree after a test run.

setup_environment! now pins XDG_CACHE_HOME to HOMEBREW_CACHE/tests right after it sandboxes HOME, mirroring how brew style already redirects RuboCop's cache to HOMEBREW_CACHE/style. A regression spec asserts the value and that it does not live under the sandboxed home.

This only steers third-party tools that honor XDG_CACHE_HOME; Homebrew's own cache is unaffected because HOMEBREW_CACHE is always exported and takes precedence over the XDG-derived default. The cache now lives outside the git tree, so a test run no longer pollutes the checkout.

---

• Have you followed our Contributing guidelines?
• Have you checked for other open Pull Requests for the same change?
• Have you explained what your changes do? Performance claims (e.g. "this is faster") must include Hyperfine benchmarks.
• Have you explained why you'd like these changes included, not just what they do?
• For bug fixes, have you given step-by-step brew commands to reproduce the bug?
• Have you written new tests (excluding integration tests)? Here's an example.
• Have you successfully run brew lgtm (style, typechecking and tests) locally?

---

• AI was used to generate or assist with generating this PR.

Claude Code (Opus 4.8) reviewed the change and suggested the explanatory comment; I wrote the fix and specs and verified with brew lgtm --online plus targeted dev-cmd/tests specs.

---

[Copilot]

Pull request overview

This pull request ensures brew tests doesn’t leave third-party tool caches inside the sandboxed test HOME (and therefore inside the working tree) by explicitly setting XDG_CACHE_HOME to a location under HOMEBREW_CACHE.

Changes:

• Pin XDG_CACHE_HOME to #{HOMEBREW_CACHE}/tests immediately after HOME is sandboxed in Homebrew::DevCmd::Tests#setup_environment!.
• Add a regression spec asserting XDG_CACHE_HOME is set as expected and is not under the sandboxed Dir.home.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
Library/Homebrew/dev-cmd/tests.rb	Sets XDG_CACHE_HOME to a cache directory under HOMEBREW_CACHE after sandboxing HOME, preventing $HOME/.cache pollution in the checkout.
Library/Homebrew/test/dev-cmd/tests_spec.rb	Adds a spec to validate setup_environment! redirects XDG_CACHE_HOME outside the sandboxed test home.

---

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

[MikeMcQuaid]

Thanks!