move docker save command from buildx to separate step

Author: leighmcculloch

.github/workflows/internal-build.yml

@@ -188,10 +188,11 @@ jobs:
         -f Dockerfile
         --target stellar-${{ matrix.dep.name }}
         -t stellar-${{ matrix.dep.name }}
-        -o type=docker,dest=/tmp/${image_filename}
         --label org.opencontainers.image.revision="${{ needs.setup.outputs.sha }}"
         ${{ steps.dep_args.outputs.args }}
         .
+    - name: Save Image
+      run: docker save stellar-${{ matrix.dep.name }} -o /tmp/image
     - name: Upload Image to Cache
       uses: actions/cache/save@v3
       id: cache