shims/super/cc: allow paths under HOMEBREW_FORMULA_BUILDPATH - #23971

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I used GPT Astra (Max) to help me root-cause this bug and iterate on the patch.

This is a fix for the build failures in Homebrew/homebrew-core#311045.

TL;DR: #23940 changes the temporary directory (HOMEBREW_TEMP) used for builds from the common temporary root to a sandbox-specific one. Specifically, this line:

https://github.com/Homebrew/brew/pull/23940/changes#diff-7eeef6e1b769c423e63e92e764feea0fc54944018bcec8c108d9a19eb550c118R584

This then interacted unintentionally with two pre-existing conditions:

1. Rust bottle builds stage outside of the sandbox-specific HOMEBREW_TEMP;
2. superenv's cc wrapper only used the sandbox-adjusted HOMEBREW_TEMP to determine which paths to retain, e.g. for -Iinclude arguments.

(2) in particular meant that, even though the sandbox had access to the staged build directory, superenv would incorrectly strip any -Iinclude that referenced it.