Implement runner fallback mechanism for docs and unit test workflows

.github/workflows/run-unit-tests.yml

@@ -11,11 +11,23 @@ concurrency:
   cancel-in-progress: true
 
 jobs:
+  pick-runner:
+    runs-on: ubuntu-latest
+    outputs:
+      runner: ${{ steps.choose.outputs.use-runner }}
+    steps:
+      - id: choose
+        uses: ./.github/actions/runner-fallback@v1
+        with:
+          primary-runner: "self-hosted,ubuntu-latest"
+          fallback-runner: "ubuntu-latest"
+          github-token: ${{ secrets.GITHUB_TOKEN }}
+
   run-unit-tests:
+    needs: pick-runner
     if: github.event.pull_request.draft == false
     name: run-unit-tests
-    # runs-on: ubuntu-latest
-    runs-on: [self-hosted, ubuntu-latest]
+    runs-on: ${{ fromJson(needs.pick-runner.outputs.runner) }}
     env:
       CARGO_TERM_COLOR: always