formula_installer: use formula from keg when post-installing source builds

[ZhongRuoyu]

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This supersedes #20988 and provides a more targeted fix for #20984; it avoids unconditionally allowing loading formulae from the API cache.

[Copilot]

Pull Request Overview

This PR removes special handling for API source cache paths in the package loading validation and extends the conditions for using keg formula paths during installation.

• Removes the conditional addition of HOMEBREW_CACHE_API_SOURCE to allowed paths in loadable_package_path?
• Adds a check to use keg formula path when building from source in post_install_formula_path
• Updates documentation comments to reflect the new build-from-source condition

Reviewed Changes

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

File	Description
Library/Homebrew/utils/path.rb	Removes API source cache path from the allowed paths when loading packages
Library/Homebrew/formula_installer.rb	Adds condition to prefer keg formula when building from source, with updated comments

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[Rylan12]

Makes sense to me!

[MikeMcQuaid]

Wonderful!