Fix-up JuliaSyntax GHA runs-on

Author: topolarity

.github/workflows/JuliaSyntaxCI.yml

@@ -17,7 +17,7 @@ concurrency:
   cancel-in-progress: ${{ startsWith(github.ref, 'refs/pull/') }}
 jobs:
   test:
-    runs-on: ${{ matrix.os }}
+    runs-on: ${{ matrix.github-runner }}
     timeout-minutes: 60
     permissions:
       contents: read