add missing 'runs-on' line to ci.yaml

Author: Techatrix

.github/workflows/ci.yaml

@@ -16,6 +16,7 @@ jobs:
       matrix:
         zig-version: [0.14.0]
         os: [ubuntu-latest, macos-latest]
+    runs-on: ${{ matrix.os }}
     steps:
       - name: Checkout
         uses: actions/checkout@v4