ci: Adapt to squash workflow

With the squash workflow, pull requests are squashed and the PR title
is used as the commit message in main. Therefore, only the PR title
needs to be conventional.

.github/workflows/test.yaml

@@ -56,5 +56,9 @@ jobs:
         run: |
           curl -sSfL https://github.com/convco/convco/releases/download/${{ env.CONVCO_VERSION }}/convco-ubuntu.zip | zcat > /usr/local/bin/convco
           chmod +x /usr/local/bin/convco
-      - name: Check
+      - name: Check pull request title
+        if: github.event_name == 'pull_request'
+        run: echo "${{ github.event.pull_request.title }}" | convco check --from-stdin
+      - name: Check all commits
+        if: github.ref == 'refs/heads/main'
         run: convco check