Formula post-install fails when building from source from API with `HOMEBREW_FORBID_PACKAGES_FROM_PATHS` set

[ZhongRuoyu]

brew doctor output

Your system is ready to brew.

Verification

• I ran brew update twice and am still able to reproduce my issue.
• My "brew doctor output" above says Your system is ready to brew or a definitely unrelated Tier message.
• This issue's title and/or description do not reference a single formula e.g. brew install wget. If they do, open an issue at https://github.com/Homebrew/homebrew-core/issues/new/choose instead.

brew config output

HOMEBREW_VERSION: 4.6.20-30-gd772896
ORIGIN: https://github.com/Homebrew/brew
HEAD: d7728966873a65a8d6fbec99890c58837d13cfc4
Last commit: 2 hours ago
Branch: main
Core tap JSON: 05 Nov 19:47 UTC
Core cask tap JSON: 05 Nov 19:47 UTC
HOMEBREW_PREFIX: /opt/homebrew
HOMEBREW_CASK_OPTS: []
HOMEBREW_DOWNLOAD_CONCURRENCY: 16
HOMEBREW_EDITOR: vim
HOMEBREW_FORBID_PACKAGES_FROM_PATHS: set
HOMEBREW_MAKE_JOBS: 8
Homebrew Ruby: 3.4.5 => /opt/homebrew/Library/Homebrew/vendor/portable-ruby/3.4.5/bin/ruby
CPU: octa-core 64-bit dunno
Clang: 17.0.0 build 1700
Git: 2.50.1 => /Applications/Xcode.app/Contents/Developer/usr/bin/git
Curl: 8.7.1 => /usr/bin/curl
macOS: 26.0.1-arm64
CLT: 26.0.0.0.1.1757719676
Xcode: 26.0
Rosetta 2: false

What were you trying to do (and why)?

This issue is filed for https://github.com/orgs/Homebrew/discussions/6512 (CC @rami3l).

Running brew install --build-from-source rustup.

The problem can be reproduced with --HEAD too; the formula just needs to be built from source.

What happened (include all command output)?

$ brew install -svd rustup      
==> Fetching downloads for: rustup
✔︎ API Source rustup.rb
✔︎ Formula rustup (1.28.2)
/opt/homebrew/Library/Homebrew/build.rb (Formulary::FromPathLoader): loading /Users/ruoyu/Library/Caches/Homebrew/api-source/Homebrew/homebrew-core/7e49f949195479ced5ab855bd6fa95f0e39fc24e/Formula/rustup.rb
[snip]
==> Cleaning
==> Finishing up
Warning: The post-install step did not complete successfully
You can try again using:
  brew postinstall rustup
Warning: Removed Sorbet lines from backtrace!
Rerun with `--verbose` to see the original backtrace
==> Homebrew requires formulae to be in a tap, rejecting:
  /Users/ruoyu/Library/Caches/Homebrew/api-source/Homebrew/homebrew-core/7e49f949195479ced5ab855bd6fa95f0e39fc24e/Formula/rustup.rb (/Users/ruoyu/Library/Caches/Homebrew/downloads/473a2db91b1c1cb9434f66652c74dc4189845670b59ac2bfa0a861ed9c9cb6cc--rustup.rb)

To create a tap, run e.g.
  brew tap-new <user|org>/<repository>
To create a formula in a tap run e.g.
  brew create <url> --tap=<user|org>/<repository>

/opt/homebrew/Library/Homebrew/utils/path.rb:38:in 'Utils::Path.loadable_package_path?'
/opt/homebrew/Library/Homebrew/formulary.rb:739:in 'Formulary::FromPathLoader.try_new'
/opt/homebrew/Library/Homebrew/formulary.rb:1457:in 'block in Formulary.loader_for'
/opt/homebrew/Library/Homebrew/formulary.rb:1456:in 'Array#each'
/opt/homebrew/Library/Homebrew/formulary.rb:1456:in 'Formulary.loader_for'
/opt/homebrew/Library/Homebrew/formulary.rb:1167:in 'Formulary.factory'
/opt/homebrew/Library/Homebrew/formula_installer.rb:1332:in 'FormulaInstaller#post_install_formula_path'
/opt/homebrew/Library/Homebrew/formula_installer.rb:1351:in 'FormulaInstaller#post_install'
/opt/homebrew/Library/Homebrew/formula_installer.rb:998:in 'FormulaInstaller#finish'
/opt/homebrew/Library/Homebrew/install.rb:490:in 'Homebrew::Install.install_formula'
/opt/homebrew/Library/Homebrew/install.rb:422:in 'block in Homebrew::Install.install_formulae'
/opt/homebrew/Library/Homebrew/install.rb:419:in 'Array#each'
/opt/homebrew/Library/Homebrew/install.rb:419:in 'Homebrew::Install.install_formulae'
/opt/homebrew/Library/Homebrew/cmd/install.rb:390:in 'Homebrew::Cmd::InstallCmd#run'
/opt/homebrew/Library/Homebrew/brew.rb:101:in '<main>'
==> Caveats
To initialize `rustup`, set a default toolchain:
  rustup default stable

If you have `rust` installed, ensure you have "$(brew --prefix rustup)/bin"
before "$(brew --prefix)/bin" in your $PATH:
  https://rust-lang.github.io/rustup/installation/already-installed-rust.html

rustup is keg-only, which means it was not symlinked into /opt/homebrew,
because it conflicts with rust.

If you need to have rustup first in your PATH, run:
  echo 'export PATH="/opt/homebrew/opt/rustup/bin:$PATH"' >> ~/.zshrc
==> Summary
🍺  /opt/homebrew/Cellar/rustup/1.28.2: 27 files, 10.9MB, built in 1 minute 22 seconds
==> Running `brew cleanup rustup`...
Disable this behaviour by setting `HOMEBREW_NO_INSTALL_CLEANUP=1`.
Hide these hints with `HOMEBREW_NO_ENV_HINTS=1` (see `man brew`).
==> Caveats
zsh completions have been installed to:
  /opt/homebrew/opt/rustup/share/zsh/site-functions

What did you expect to happen?

The formula's post-install step should be successfully run.

Step-by-step reproduction instructions (by running brew commands)

HOMEBREW_FORBID_PACKAGES_FROM_PATHS=1 brew install -svd rustup