Change event to pull_request.head.sha

Author: ndavidova

.github/workflows/testing-farm.yaml

@@ -105,6 +105,6 @@ jobs:
           pull_request_status_name: "Fedora container upgrade test"
           update_pull_request_status: "true"
           create_github_summary: "true"
-          variables: "REPO_URL=${{ github.server_url }}/${{ github.repository }};REPO_NAME=${{ github.repository }};PR_NUMBER=${{ github.event.issue.number }};"
+          variables: "REPO_URL=${{ github.server_url }}/${{ github.repository }};REPO_NAME=${{ github.repository }};PR_NUMBER=${{ github.event.pull_request.head.sha }};"