Add trigger event sha

Author: jiechen0826

.github/workflows/ci.yml

@@ -21,6 +21,24 @@ env:
   LINUX_BOOT_FILES_PATH: ${{ github.workspace }}/LinuxBootFiles
 
 jobs:
+  find-sha:
+    runs-on: ubuntu-latest
+    steps: 
+    - name: Find SHA of the triggering workflow (i.e. PR)
+      run: |
+        if [ "$EVENT" == 'pull_request' ]
+        then
+          echo "::set-output name=sha::$(echo ${{github.event.pull_request.head.sha}})"
+        elif [ "$EVENT" == 'workflow_run' ]
+        then
+          echo "::set-output name=sha::$(echo ${{github.event.workflow_run.head_sha}})"
+        else
+          echo "::set-output name=sha::$(echo $GITHUB_SHA)"
+        fi
+    env:
+      EVENT: ${{ github.event_name }}
+      REF: ${{ github.ref }}
+
   lint:
     runs-on: "windows-2022"
     strategy: