fix: resolve ci.yaml failures on push events

Author: westarle

.github/workflows/ci.yaml

@@ -203,8 +203,8 @@ jobs:
     - run: .kokoro/build.sh
       env:
         JOB_TYPE: lint
-        HEAD_SHA: ${{ github.event.pull_request.head.sha }}
-        BASE_SHA: ${{ github.event.pull_request.base.sha }}
+        HEAD_SHA: ${{ github.event.pull_request.head.sha || github.sha }}
+        BASE_SHA: ${{ github.event.pull_request.base.sha || github.event.before }}
   enforcer:
     runs-on: ubuntu-latest
     steps: