add cache restore key and run gc in post step

This will allow us to restore all available cached hooks to reduce the
time spent "Initializing environment" on each run.

If any of the hooks are updated, the `pre-commit gc` command will remove
the now-unused hooks from ~/.cache/pre-commit, ensuring that we don't
store them in the cache indefinitely.

action.yml

@@ -16,5 +16,10 @@ runs:
     with:
       path: ~/.cache/pre-commit
       key: pre-commit-3|${{ env.pythonLocation }}|${{ hashFiles('.pre-commit-config.yaml') }}
+      restore-keys: |
+        pre-commit-3|${{ env.pythonLocation }}|
+        pre-commit-3|
   - run: pre-commit run --show-diff-on-failure --color=always ${{ inputs.extra_args }}
     shell: bash
+  - run: pre-commit gc
+    shell: bash