feat: Add external PR safe to test support (#145)

Use pull_request_target trigger condition to support external PR and use
`ok-to-test` label to allow the CI running.

Author: yaalsn

.github/workflows/acctest.yml

@@ -13,10 +13,17 @@
 # limitations under the License.
 
 name: Tests
-on: [pull_request]
+on:
+  pull_request:
+  pull_request_target:
+    types: [labeled]
+
 jobs:
   acctest:
     name: Run acceptance tests
+    if: |
+      (github.event_name == 'pull_request' && github.event.pull_request.head.repo.full_name == github.repository) ||
+      (github.event_name == 'pull_request_target' && contains(github.event.pull_request.labels.*.name, 'ok-to-test'))
     runs-on: ubuntu-latest
     env:
       ACC_TEST_SERVICE_ACCOUNT: ${{ secrets.ACC_TEST_SERVICE_ACCOUNT }}
@@ -27,6 +34,8 @@ jobs:
     steps:
       - name: Checkout Code
         uses: actions/checkout@v3
+        with:
+          ref: ${{ github.event.pull_request.head.sha }}
 
       - name: Setup Go
         uses: actions/setup-go@v3