CI: dont run aarch64-mac workflow on forks

The workflow uses a self hosted runner which is only available for
hexops/mach* repositories. Without this commit, whenever a fork of mach
or mach-* were pushed, it would queue an aarch64-mac job and then crash
after one day due to no machines being found.

.github/workflows/m1_ci.yml

@@ -7,6 +7,7 @@ on:
 jobs:
   aarch64-macos:
     if: ${{ github.event.workflow_run.conclusion == 'success' }}
+    if: github.repository == 'hexops/mach-gpu-dawn'
     runs-on: [self-hosted, macOS, ARM64]
     defaults:
       run: