Add a missing runs-on field (#5562)

.github/workflows/continous-integration-os.patch.yml

@@ -16,6 +16,9 @@ on:
 jobs:
   test:
     name: Test ${{ matrix.rust }} on ${{ matrix.os }}${{ matrix.features }}
+    # We're just doing this job for the name, the platform doesn't matter.
+    # So we use the platform with the most concurrent instances.
+    runs-on: ubuntu-latest
     strategy:
       matrix:
         # TODO: Windows was removed for now, see https://github.com/ZcashFoundation/zebra/issues/3801