chore: add native deduplication to flake checks

[weriomat]

this should fix #318 by deduplicating the flake paths with a Hashset

[Sntx626]

This does not de-duplicate all flake checks, just flake checks from having the same node provided multiple times...

What is the use-case for this?

[weriomat]

Currently, if I deploy multiple nodes from a single repo (i.e. deploy --targets .#a .#b) it should run nix flake check . for each node/ path for the node. Thus, it should be run two times, my attempt is to deduplicate by the path such as it will only get run once regardless of the amount of nodes deployed from that repo.
That is my understanding of the issue described in #318.
Have I missed something?