Just skip the job if not on ruby/ruby

Using the same `runs-os` across all jobs in the same workflow makes it
more likely to pass in case one of these runners is experiencing an
outage.

macos-arm-oss is generally not available outside ruby/ruby, so it's
fair to skip it for forks.

.github/workflows/yjit-macos.yml

@@ -108,9 +108,9 @@ jobs:
         if: ${{ failure() }}
 
   result:
-    if: ${{ always() }}
+    if: ${{ always() && github.repository == 'ruby/ruby' }}
     name: ${{ github.workflow }} result
-    runs-on: ubuntu-latest
+    runs-on: macos-arm-oss
     needs: [make]
     steps:
       - run: exit 1