ci: exclude dependbot from running tests on s390x machine (#5266)

.github/workflows/ci.yml

@@ -143,7 +143,7 @@ jobs:
   s390x-test:
     name: test (s390x on IBM Z)
     runs-on: ubuntu-latest
-    if: github.event.pull_request.head.repo.full_name == github.repository
+    if: github.event.pull_request.head.repo.full_name == github.repository && github.actor != 'dependabot[bot]'
     continue-on-error: true  # August 2020: s390x VM is down due to weather and power issues
     steps:
       - name: Checkout code into the Go module directory