CI: fix runtime testing for non master branch

The runtime testing always ran on master branch aka snapshots since the
branch wasn't passed over to the container execution!

Signed-off-by: Paul Spooren <mail@aparcar.org>

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

@@ -160,7 +160,7 @@ jobs:
         run: |
           docker build -t test-container --build-arg ARCH .github/workflows/
         env:
-          ARCH: ${{ matrix.arch }}
+          ARCH: ${{ matrix.arch }}-${{ env.BRANCH }}
 
       - name: Test via Docker container
         if: ${{ matrix.runtime_test }}