include gem_home perms workaround

Remove write access to the world on GEM_HOME. Hopefully a temporary
workaround until the runner image is fixed.

See actions/runner-images#10215

.github/actions/test_gem/action.yml

@@ -82,6 +82,13 @@ runs:
         ruby-version: "${{ inputs.ruby }}"
         bundler:  "latest"
         working-directory: "${{ steps.setup.outputs.gem_dir }}"
+    # Perms workaround. See https://github.com/actions/runner-images/issues/10215
+    - name: Fix GEM_HOME permissions on GitHub Actions Runner
+      if: "${{ steps.setup.outputs.appraisals == 'true' }}"
+      shell: bash
+      run: |
+        # 🛠️😭 Fix GEM_HOME permissions 😭🛠️
+        chmod -R o-w $(gem env home)
     - name: Install dependencies and generate appraisals
       if: "${{ steps.setup.outputs.appraisals == 'true' }}"
       shell: bash