ci: don't run s390x tests on PRs of forks (#3494)

* ci: don't run s390x tests on PRs of forks

* ci: check if fork by matchinging name from event against name of repo

.github/workflows/ci.yml

@@ -123,7 +123,7 @@ jobs:
   s390x-test:
     name: test (s390x on IBM Z)
     runs-on: ubuntu-latest
-    if: ${{ github.repository == 'caddyserver/caddy' }}
+    if: github.event.pull_request.head.repo.full_name == github.repository
     steps:
       - name: Checkout code into the Go module directory
         uses: actions/checkout@v2