The arm64 Homebrew xcodegen bottle is corrupt on current runner images
(pour fails with 'Cellar/xcodegen/<ver> is not a directory'), and
reinstalling re-pours the same broken tarball. Use the repo's pinned,
checksummed scripts/install-xcodegen.sh like ci.yml already does.
Periphery scans fail org-wide when the runner image ships a broken
Cellar entry (pour errors with 'Cellar/xcodegen/<ver> is not a
directory'). Pre-clean the path and retry the install once.
