Build pipeline: Don't use a dev build of the test frontend for acceptance tests

[spruce-bruce]

The build pipeline is configured to start the test frontend using dev mode. That means that there is a file watcher running and it's forcing us to complicate our configs with a CHOKIDAR_USEPOLLING flag. Let's use a real production build of the test frontend instead, and let's remove that CHOKIDAR_USEPOLLING flag.

https://github.com/synapsestudios/oidc-platform/blob/master/.github/workflows/build.yml#L195

• The test frontend can be built in parallel to the oidc image
• the build can be cached. We can either use an old artifact or some other cache when there are no changes to the test FE.