CI: Add actions:write to gem-publish workflow

The last step in this workflow's publish run opens a PR, but the actions
are not running on the PR, requiring a manual triggering or force-push.
It will save us time if this way, we can merge the created PR sooner.

.github/workflows/publish.yml

@@ -8,6 +8,7 @@ jobs:
     name: Publish to RubyGems
     runs-on: ubuntu-latest
     permissions:
+      actions: write
       contents: write
       id-token: write
       pull-requests: write