Use absolute brew path in container setup

Author: eleanorjboyd

.github/workflows/pr-check.yml

@@ -417,8 +417,8 @@ jobs:
           # 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'
+          su - linuxbrew -c '/home/linuxbrew/.linuxbrew/bin/brew update || true'
+          su - linuxbrew -c '/home/linuxbrew/.linuxbrew/bin/brew install python@3.12 python@3.11'
         shell: bash
 
       # Rust