Fix the release script for macOS

`date --utc` is not POSIX and macOS doesn't have it.
`-u` is the standard equivalent.

.github/build-release.sh

@@ -6,6 +6,6 @@ rm -rf build/
 export PUBLIC_URL=/admin-ui
 CI=false npm run build
 
-FILENAME="oc-admin-ui-$(date --utc +%F).tar.gz"
+FILENAME="oc-admin-ui-$(date -u +%F).tar.gz"
 cd build
 tar -czf ../$FILENAME *