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firefox: make nativeMessagingHosts unique
In case of duplicated messaging hosts, path collisions make the firefox drv fail to build.
Repro:
```nix
pkgs.firefox.override { nativeMessagingHosts = [ pkgs.passff-host pkgs.passff-host ]; }
```
This may happen e.g. if `programs.firefox.nativeMessagingHosts.packages` is
set in multiple locations with duplicated messaging hosts.
Calling `lib.unique` resolves the issue if an identical messaging host is duplicated,
but does correctly throw if multiple different native messaging host
packages try to provide the same native messaging host interface.