brew.sh: work around non-writable cache for lgtm commands.

[MikeMcQuaid]

Under some e.g. AI sandboxes HOMEBREW_CACHE is not accessible but the current working directory's tmp is instead. In those cases, print a warning and automatically copy and use the other cache.

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[Copilot]

Pull request overview

This PR adds a workaround in brew.sh for environments where HOMEBREW_CACHE is not writable (e.g., some sandboxed/AI environments), automatically falling back to a repository-local cache and copying any existing API cache to avoid permission-related failures in brew lgtm-related workflows.

Changes:

• Add a cache-writability check in brew.sh for audit/lgtm/style/tests, falling back to $HOMEBREW_REPOSITORY/tmp/cache and copying api/ cache contents.
• Add an RSpec integration-style test covering the cache fallback behavior and warning output.
• Remove sandbox setup instructions from AGENTS.md that are now intended to be handled automatically.

Reviewed changes

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

File	Description
Library/Homebrew/brew.sh	Implements the cache fallback + warning + best-effort API cache copy for selected commands.
Library/Homebrew/test/dev-cmd/lgtm_spec.rb	Adds a spec that exercises the fallback path by invoking an isolated brew lgtm --help.
AGENTS.md	Removes manual sandbox cache-copy instructions now superseded by the script behavior.

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