test: ensure os is returned from setup

Author: pieh

.github/workflows/run-tests.yml

@@ -16,6 +16,7 @@ jobs:
     runs-on: ubuntu-latest
     outputs:
       matrix: ${{ steps.set-matrix.outputs.matrix }}
+      os: ${{ steps.set-matrix.outputs.os }}
     steps:
       - name: Check PR labels
         if: github.event_name == 'pull_request'