oscar: Add musicbrainz plugin to beets

This used to be in core but got moved to a plugin earlier this year:

https://github.com/beetbox/beets/pull/5748

This was released as part of Beets 2.4.0, which I've finally upgrade to:

https://docs.beets.io/en/latest/changelog.html#september-13-2025

> [MusicBrainz Plugin](https://docs.beets.io/en/latest/plugins/musicbrainz.html):
> The MusicBrainz autotagger has been moved to a separate plugin. The
> default [plugins](https://docs.beets.io/en/latest/reference/config.html#plugins-config)
> includes `musicbrainz`, but if you’ve customized your `plugins` list
> in your configuration, you’ll need to explicitly add `musicbrainz` to
> continue using this functionality. Configuration option
> [`enabled`](https://docs.beets.io/en/latest/plugins/musicbrainz.html#conf-enabled)
> has thus been deprecated.
> [#2686](https://github.com/beetbox/beets/issues/2686)
> [#4605](https://github.com/beetbox/beets/issues/4605)

oscar/configuration.nix

@@ -320,6 +320,7 @@
           "fetchart"
           # "lyrics"
           # "subsonicupdate" # TODO
+          "musicbrainz"
           "mbsubmit"
           "edit"
         ];