cgal: ensure Find* modules look inside HOMEBREW_PREFIX

[carlocab]

• Have you followed the guidelines for contributing?
• Have you ensured that your commits follow the commit style guide?
• Have you checked that there aren't other open pull requests for the same formula update/change?
• Have you built your formula locally with HOMEBREW_NO_INSTALL_FROM_API=1 brew install --build-from-source <formula>, where <formula> is the name of the formula you're submitting?
• Is your test running fine brew test <formula>, where <formula> is the name of the formula you're submitting?
• Does your build pass brew audit --strict <formula> (after doing HOMEBREW_NO_INSTALL_FROM_API=1 brew install --build-from-source <formula>)? If this is a new formula, does it pass brew audit --new <formula>?

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This also helps guarantee that we build an :all bottle.

[carlocab]

All checksums match ✅

70d4bde9024c3e8215eaeba6043275bbd71e7b5635d36710515726808c60db09
70d4bde9024c3e8215eaeba6043275bbd71e7b5635d36710515726808c60db09
70d4bde9024c3e8215eaeba6043275bbd71e7b5635d36710515726808c60db09
70d4bde9024c3e8215eaeba6043275bbd71e7b5635d36710515726808c60db09
70d4bde9024c3e8215eaeba6043275bbd71e7b5635d36710515726808c60db09
70d4bde9024c3e8215eaeba6043275bbd71e7b5635d36710515726808c60db09

[github-actions]

🤖 An automated task has requested bottles to be published to this PR.