build(cypress): Use pull_request_target event to run cypress (#11750)

* Use pull_request_target event to run cypress in order to access repo secrets

* Summary job for branch protection requirements

* Restore pull_request trigger

* Use merge base for e2e tests

* Restore push trigger for all branches

.github/workflows/superset-e2e.yml

@@ -1,13 +1,18 @@
 name: E2E
 
-on: [push, pull_request]
+on: [push, pull_request, pull_request_target]
 
 jobs:
-  Cypress:
+  cypress-matrix:
     runs-on: ubuntu-18.04
     strategy:
-      fail-fast: true
+      # when one test fails, DO NOT cancel the other
+      # containers, because this will kill Cypress processes
+      # leaving the Dashboard hanging ...
+      # https://github.com/cypress-io/github-action/issues/48
+      fail-fast: false
       matrix:
+        containers: [1, 2, 3]
         browser: ["chrome"]
     env:
       FLASK_ENV: development
@@ -34,8 +39,14 @@ jobs:
         uses: styfle/cancel-workflow-action@0.6.0
         with:
           access_token: ${{ github.token }}
-      - name: Checkout code
+      - name: Checkout code (push)
+        if: github.event_name == 'push'
         uses: actions/checkout@v2
+      - name: Checkout code (pull_request)
+        if: github.event_name == 'pull_request' || github.event_name == 'pull_request_target'
+        uses: actions/checkout@v2
+        with:
+          ref: 'refs/pull/${{ github.event.number }}/merge'
       - name: Setup Python
         uses: actions/setup-python@v2
         with:
@@ -89,3 +100,12 @@ jobs:
         with:
           name: screenshots
           path: ${{ github.workspace }}/superset-frontend/cypress-base/cypress/screenshots
+  Cypress:
+    if: ${{ always() }}
+    name: Cypress (chrome)
+    runs-on: ubuntu-18.04
+    needs: cypress-matrix
+    steps:
+      - name: Check build matrix status
+        if: ${{ needs.cypress-matrix.result != 'success' }}
+        run: exit 1