set bundle path instead of chmod'ing sys gem path

Alternative workaround to /opt/hostedtoolcache/Ruby being world writable
in the ubuntu 20240708.1.0 GA runner image.

See actions/runner-images#10215

.github/actions/test_gem/action.yml

@@ -82,18 +82,12 @@ 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
       run: |
         # 💎 Install dependencies and generate appraisals 💎
+        bundle config path ${HOME}/.gem/ruby/${{ inputs.ruby }}
         bundle install --quiet --jobs=3 --retry=4
         bundle exec appraisal generate
       working-directory: "${{ steps.setup.outputs.gem_dir }}"
@@ -105,6 +99,7 @@ runs:
         if [[ -f "Appraisals" ]]; then
           for i in `bundle exec appraisal list | sed 's/-/_/g' `; do
             echo "::group::🔎 Appraising ${i}"
+            bundle config path ${HOME}/.gem/ruby/${{ inputs.ruby }}
             BUNDLE_GEMFILE=gemfiles/${i}.gemfile bundle install --quiet --jobs=3 --retry=4 && \
             BUNDLE_GEMFILE=gemfiles/${i}.gemfile bundle show && \
             BUNDLE_GEMFILE=gemfiles/${i}.gemfile bundle exec rake test || exit