ci: fix expression in build workflow (#42)

Signed-off-by: Mike Beaumont <[email protected]>

.github/workflows/build.yaml

@@ -20,6 +20,7 @@ permissions:
 
 env:
   DOCKER_REPO: kumahq/kuma-demo
+  PUSH_IMAGE: ${{ toJSON(github.event_name == 'push' || fromJSON(inputs.push)) }}
 
 jobs:
   build-and-push:
@@ -30,14 +31,14 @@ jobs:
       - name: Set up Docker Buildx
         uses: docker/setup-buildx-action@v3
       - name: Login to Docker Hub
-        if: ${{ fromJSON(inputs.push) }}
+        if: fromJSON(env.PUSH_IMAGE)
         uses: docker/login-action@v3
         with:
           username: ${{ vars.DOCKER_USERNAME }}
           password: ${{ secrets.DOCKER_API_KEY }}
       - name: Build and push
         uses: docker/build-push-action@v5
         with:
-          push: ${{ github.event_name == 'push' || fromJSON(inputs.push) }}
+          push: ${{ fromJSON(env.PUSH_IMAGE) }}
           platforms: linux/amd64, linux/arm64
           tags: ${{ env.DOCKER_REPO }}:${{ github.event_name == 'push' && 'dev' || inputs.tag }}