Fix self-hosted mac runner

Author: ElliottKasoar

.github/workflows/mac.yml

@@ -5,7 +5,7 @@ on: [push, pull_request]
 jobs:
 
   tests:
-    runs-on: self-hosted
+    runs-on: [self-hosted, macOS]
     if: github.repository == 'stfc/janus-core'
     timeout-minutes: 30
     strategy: