Skip the post run step entirely if the cache is disabled

Previously if the `cache: false` (the default for this Action), the
Action's "post run" step would still be executed.

Whilst this step was fast (since it returned early if the cache was
disabled), it still causes unnecessary noise in the job's steps list.

For example as seen in:
https://github.com/pypa/get-pip/actions/runs/8679713478/job/23798960684

Now, the post run step is skipped if the cache is disabled, thanks
to the `post-if` syntax supporting the `github.events.inputs.*`
context.

See:
https://docs.github.com/en/actions/creating-actions/metadata-syntax-for-github-actions#runspost-if
https://github.com/actions/cache/blob/0c45773b623bea8c8e75f6c82b208c3cf94ea4f9/action.yml#L40

action.yml

@@ -37,7 +37,7 @@ runs:
   using: 'node20'
   main: 'dist/setup/index.js'
   post: 'dist/cache-save/index.js'
-  post-if: success()
+  post-if: 'success() && github.event.inputs.cache'
 branding:
   icon: 'code'
   color: 'yellow'