export tarball image file to be compatible with docker load

`docker load` expects a file exported from `docker save`, but
`docker/build-push-action` exports tarballs with
`--output type=oci,dest=` for now, so we need to pass
`type=docker` explicitly

Author: kateinoigakukun

.github/workflows/build.yml

@@ -57,7 +57,7 @@ jobs:
         uses: docker/build-push-action@v3
         with:
           context: builders/${{ matrix.entry.tag }}
-          outputs: type=oci,dest=/tmp/image.tar
+          outputs: type=docker,dest=/tmp/image.tar
           cache-from: type=local,src=/tmp/.buildx-cache
           cache-to: type=local,dest=/tmp/.buildx-cache-new
       - name: Upload image artifact