ci: Skip build steps completely on releases

No point in running these steps if we are not pushing anyway. The
release workflow will take care.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>

.github/workflows/master.yml

@@ -29,18 +29,20 @@ jobs:
           deploy-token: ${{ secrets.PACKAGE_DEPLOY_TOKEN }}
       - name: Build kas image
         uses: docker/build-push-action@v5
+        if: ${{ env.PUSH_MASTER }}
         with:
           cache-from: type=gha
           cache-to: type=gha,mode=max
           platforms: linux/amd64,linux/arm64
-          push: ${{ env.PUSH_MASTER }}
+          push: true
           tags: ghcr.io/siemens/kas/kas
       - name: Build kas-isar image
         uses: docker/build-push-action@v5
+        if: ${{ env.PUSH_MASTER }}
         with:
           cache-from: type=gha
           cache-to: type=gha,mode=max
           target: kas-isar
           platforms: linux/amd64,linux/arm64
-          push: ${{ env.PUSH_MASTER }}
+          push: true
           tags: ghcr.io/siemens/kas/kas-isar