ci: skip entire autoland job for non-bot users (#382)

In contributor PRs such as #372 we see that the pr/autoland job fails
due to permissions issues obtaining an app token from a fork.

This job is actually only required to auto-land bot PRs. Instead of
skipping just the auto-land setting step, we should be skipping the
entire thing.

Fixes #381

.github/workflows/pr.yml

@@ -5,6 +5,7 @@ on:
   pull_request:
 jobs:
   automerge:
+    if: github.actor == 'cadobot[bot]' && !github.event.pull_request.draft
     runs-on: ubuntu-latest
     steps:
       - name: Generate Bot Token
@@ -14,7 +15,6 @@ jobs:
           app_id: ${{ secrets.APP_ID }}
           private_key: ${{ secrets.APP_PRIVATE_KEY }}
       - name: Automerge Bot Pull Requests
-        if: github.actor == 'cadobot[bot]' && !github.event.pull_request.draft
         run: gh pr merge --auto --squash "${{ github.event.pull_request.html_url }}"
         env:
           GITHUB_TOKEN: ${{ steps.bot.outputs.token }}