squash: use a separate cache key

.github/actions/test_gem/action.yml

@@ -57,7 +57,7 @@ runs:
         # of the benefits of bundler caching.
         rm -f "${dir}/Gemfile.lock"
 
-        echo "cache_key=mri" >> $GITHUB_OUTPUT
+        echo "cache_key=cruby" >> $GITHUB_OUTPUT
         if [[ "${{ inputs.ruby }}" == "jruby" ]]; then
           echo "cache_key=jruby" >> $GITHUB_OUTPUT
         elif [[ "${{ inputs.ruby }}" == "truffleruby" ]]; then