fix workflow syntax error (#4301)

.github/workflows/build-multiarch.yaml

@@ -13,7 +13,6 @@ on:
       - 'release-*'
 
 jobs:
-  if: github.event_name == 'push'
   build:
     name: Build
     runs-on: ubuntu-latest
@@ -37,6 +36,7 @@ jobs:
         env:
           DOCKER_USERNAME: ${{ secrets.DOCKER_USERNAME }}
           DOCKER_PASSWORD: ${{ secrets.DOCKER_PASSWORD }}
+        if: github.event_name == 'push'
         run: |
           echo ${{ secrets.DOCKER_PASSWORD }} | docker login -u ${{ secrets.DOCKER_USERNAME }} --password-stdin
           REPO=kubespheredev TAG="${GITHUB_REF#refs/*/}" make container-cross-push