nixos/gonic: allow gonic to perform non-local DNS resolution

Gonic accesses external services (e.g. Listenbrainz or last.FM) for
scrobbling, but it was previously not allowed to read
`/etc/resolv.conf`.

This had the effect that, unless a local resolver was configured on
the system, any connection attempt would fail due to DNS resolution
being unavailable.

nixos/modules/services/audio/gonic.nix

···
         ReadWritePaths = "";
         BindReadOnlyPaths = [
           # gonic can access scrobbling services
+          "-/etc/resolv.conf"
           "-/etc/ssl/certs/ca-certificates.crt"
           builtins.storeDir
           cfg.settings.podcast-path