build: Only check DCO on PRs

The DCO tool doesn't understand merge_groups but we still need to have a
valid status check to allow the merge group to proceed.

Signed-off-by: Rob Bradford <rbradford@rivosinc.com>

.github/workflows/dco.yaml

@@ -12,6 +12,7 @@ jobs:
       with:
         python-version: '3.x'
     - name: Check DCO
+      if: ${{ github.event_name == 'pull_request' }}
       env:
         GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
       run: |