nginx: enable health check at a different location than `/`

### Enhancement Proposal

Currently, in https://github.com/canonical/cos-coordinated-workers/blob/main/src/coordinated_workers/nginx.py#L588, we set a "heath" endpoint on nginx at the root `/` location, which, IMO, is not ideal.
Perhaps we should put it in a `/health` and verify that this change doesn't break loki and mimir.

Start in src/coordinated_workers/nginx.py around line 588, where the health endpoint is currently configured at /. Check how the nginx location is used by Loki and Mimir, then determine whether moving it to /health preserves their behavior. Done means the endpoint uses the new location without breaking Loki or Mimir.

Written by the indexing model from the issue text.