multi-arch-test-build: run it only after the Formality check succeeds

Most of the time, when people want to contribute to the repository for the first time
or after a longer break, the multi-arch-test-build passes, but the formality check fails
 and requires the user to make some changes.

This puts quite a load on the CI/CD, since the multi-arch-test-build runs again even though it already passed before.

Let’s now set it up so that the formality check is resolved first, and only then the package tests are executed.

Signed-off-by: Josef Schlehofer <[email protected]>

Author: BKPepe

.github/workflows/multi-arch-test-build.yml

@@ -1,9 +1,13 @@
 name: Feeds Package Test Build
 
 on:
-  pull_request:
+  workflow_run:
+    workflows: ["Test Formalities"]
+    types:
+      - completed
 
 jobs:
   build:
     name: Feeds Package Test Build
+    if: github.event.workflow_run.conclusion == 'success'
     uses: openwrt/actions-shared-workflows/.github/workflows/multi-arch-test-build.yml@main