Update integration test workflow

Signed-off-by: Yi Chen <[email protected]>

.github/workflows/integration.yaml

@@ -93,6 +93,18 @@ jobs:
   build-helm-chart:
     runs-on: ubuntu-20.04
     steps:
+      - name: Determine branch name
+        id: get_branch
+        run: |
+          BRANCH=""
+          if [ "${{ github.event_name }}" == "push" ]; then
+            BRANCH=${{ github.ref }}
+          elif [ "${{ github.event_name }}" == "pull_request" ]; then
+            BRANCH=${{ github.base_ref }}
+          fi
+          echo "Branch name: $BRANCH"
+          echo "BRANCH=$BRANCH" >> "$GITHUB_OUTPUT"
+
       - name: Checkout source code
         uses: actions/checkout@v4
         with:
@@ -122,8 +134,10 @@ jobs:
 
       - name: Run chart-testing (list-changed)
         id: list-changed
+        env:
+          BRANCH: ${{ steps.get_branch.outputs.BRANCH }}
         run: |
-          changed=$(ct list-changed --target-branch ${{ github.ref }})
+          changed=$(ct list-changed --target-branch $BRANCH)
           if [[ -n "$changed" ]]; then
             echo "changed=true" >> "$GITHUB_OUTPUT"
           fi