workflows/premerge: Fix condition for macos job (llvm#125237)

Author: tstellar

.github/workflows/premerge.yaml

@@ -139,8 +139,9 @@ jobs:
       group: ${{ github.workflow }}-macos-${{ github.event.pull_request.number || github.sha }}
       cancel-in-progress: true
     if: >-
+      github.repository_owner == 'llvm' &&
       (startswith(github.ref_name, 'release/') ||
-       startswith(github.base_ref, 'refs/heads/release/'))
+       startswith(github.base_ref, 'release/'))
     steps:
       - name: Checkout LLVM
         uses: actions/checkout@v4