fix: GH action env-var usage (#34)

.github/workflows/maven.yml

@@ -34,7 +34,7 @@ jobs:
       - name: Set up Docker Buildx
         uses: docker/setup-buildx-action@v2
         with:
-          platforms: $BUILDX_PLATFORMS
+          platforms: ${{ env.BUILDX_PLATFORMS }}
 
 #      - name: Build & Push (ubuntu)
 #        run: make