Fix Homebrew container install user

Author: eleanorjboyd

.github/workflows/pr-check.yml

@@ -414,10 +414,11 @@ jobs:
       - name: Homebrew Python
         if: startsWith( matrix.image, 'homebrew')
         run: |
-          # homebrew/brew:4.4.6 broke running `brew install` as root.
-          # As a workaround, running `brew update` and ignoring errors coming from it fixes `brew install`.
-          brew update || true
-          brew install python@3.12 python@3.11
+          # The job container runs as root so Actions checkout/tooling can write to the
+          # mounted workspace, but Homebrew's prefix is owned by linuxbrew and recent
+          # homebrew/brew images have been brittle when `brew install` runs as root.
+          su - linuxbrew -c 'brew update || true'
+          su - linuxbrew -c 'brew install python@3.12 python@3.11'
         shell: bash
 
       # Rust