Preflight workflow (#5692)

Preflight reusable workflow - aggregated workflow to perform all
preliminary jobs
It currently includes:
- check changed files
- `set-image` (and runner) job
- useful variables

.github/workflows/build-misc.yml

@@ -16,34 +16,16 @@ permissions:
   contents: read
 
 jobs:
-  set-image:
-    # GitHub Actions allows using 'env' in a container context.
-    # However, env variables don't work for forks: https://github.com/orgs/community/discussions/44322
-    # This workaround sets the container image for each job using 'set-image' job output.
-    runs-on: ubuntu-latest
-    outputs:
-      IMAGE: ${{ steps.set_image.outputs.IMAGE }}
-      RUNNER: ${{ steps.set_runner.outputs.RUNNER }}
-    steps:
-      - name: Checkout
-        uses: actions/checkout@v4
-      - id: set_image
-        run: cat .github/env >> $GITHUB_OUTPUT
-      - id: set_runner
-        run: |
-          # Run merge queues on persistent runners
-          if [[ $GITHUB_REF_NAME == *"gh-readonly-queue"* ]]; then
-            echo "RUNNER=arc-runners-polkadot-sdk-beefy-persistent" >> $GITHUB_OUTPUT
-          else
-            echo "RUNNER=arc-runners-polkadot-sdk-beefy" >> $GITHUB_OUTPUT
-          fi
+
+  preflight:
+    uses: ./.github/workflows/reusable-preflight.yml
 
   build-runtimes-polkavm:
     timeout-minutes: 20
-    needs: [set-image]
-    runs-on: ${{ needs.set-image.outputs.RUNNER }}
+    needs: [preflight]
+    runs-on: ${{ needs.preflight.outputs.RUNNER }}
     container:
-      image: ${{ needs.set-image.outputs.IMAGE }}
+      image: ${{ needs.preflight.outputs.IMAGE }}
     steps:
       - name: Checkout
         uses: actions/checkout@v4
@@ -64,10 +46,10 @@ jobs:
 
   build-subkey:
     timeout-minutes: 20
-    needs: [set-image]
-    runs-on: ${{ needs.set-image.outputs.RUNNER }}
+    needs: [preflight]
+    runs-on: ${{ needs.preflight.outputs.RUNNER }}
     container:
-      image: ${{ needs.set-image.outputs.IMAGE }}
+      image: ${{ needs.preflight.outputs.IMAGE }}
     steps:
       - name: Checkout
         uses: actions/checkout@v4