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.

0 files changed, 0 insertions, 0 deletions