Fix ensuring the release branch in workflow

.github/workflows/build-app.yaml

@@ -48,6 +48,10 @@ jobs:
     runs-on: ubuntu-latest
     container: ghcr.io/syncthing/syncthing-android-builder
     steps:
+      - uses: actions/checkout@v3
+        with:
+          submodules: true
+
       - name: Ensure release branch
         run: |
           if ! git branch --contains $(git rev-parse HEAD) | grep release >/dev/null; then
@@ -55,10 +59,6 @@ jobs:
             exit 1
           fi
 
-      - uses: actions/checkout@v3
-        with:
-          submodules: true
-
       - name: build_release
         env:
           SYNCTHING_RELEASE_KEY_ALIAS: android