Add unionvisor to nixos module (#912)

* feat(unionvisor): add binary to environment.systemPackages

* fix: move environment.systemPackages into config

* fix: remove trailing backslash

* fix: wrap unionvisor directly instead of through cfg.bundle

* fix: wrapProgram unionvisor

* fix: use $out/bin/unionvisor

* fix: set root via env var

* fix: unionvisor is in $out/unionvisor

* fix: move unionvisor to $out/bin/

* chore: fmt