nixos/grafana: Filter out duplicate plugins

If the same plugin appears multiple times in `declarativePlugins`, for
example due to being added both by a module and in user config, the
build fails with an error message similar to

ln: failed to create symbolic link 'grafana-worldmap-panel/glmqcj88zk2bz3mvdr3r7920wxg02qnq-grafana-worldmap-panel-0.3.2': Permission denied

This is solved by removing all duplicates.

nixos/modules/services/monitoring/grafana.nix

···
       defaultText = "pkgs.grafana";
       type = types.package;
     };
+
     declarativePlugins = mkOption {
       type = with types; nullOr (listOf path);
       default = null;
       description = "If non-null, then a list of packages containing Grafana plugins to install. If set, plugins cannot be manually installed.";
       example = literalExample "with pkgs.grafanaPlugins; [ grafana-piechart-panel ]";
+      # Make sure each plugin is added only once; otherwise building
+      # the link farm fails, since the same path is added multiple
+      # times.
+      apply = x: if isList x then lib.unique x else x;
     };
 
     dataDir = mkOption {