nixos/grafana: Filter out duplicate plugins
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.

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