chore(ci): Verify building of cross images for PRs (#5910)

Author: ovr

.github/workflows/push-cross-images.yml

@@ -5,6 +5,12 @@ on:
     paths:
       - '.github/workflows/push-cross-images.yml'
       - 'rust/cubestore/cross/**'
+    branches:
+      - 'master'
+  pull_request:
+    paths:
+      - '.github/workflows/push-cross-images.yml'
+      - 'rust/cubestore/cross/**'
 
 jobs:
   docker-dev:
@@ -22,6 +28,7 @@ jobs:
       - name: Checkout
         uses: actions/checkout@v2
       - name: Login to DockerHub
+        if: ${{ github.ref == 'refs/heads/master' }}
         uses: docker/login-action@v1
         with:
           username: ${{ secrets.DOCKERHUB_USERNAME }}