Performance impact

[PassiveLemon]

This is related to #159 since the performance impacts of nh are still present and I've heard from others that also have this issue. I don't have any new benchmarks to add yet, but I will try to set some up soon.

[llakala]

I believe this is because of https://github.com/maralorn/nix-output-monitor#the-easy-way - nh uses the Nom subcommands, which double eval time. as they run the internal nix commands twice.

[viperML]

Nom doesn't call nix internally in the current nh implementation.

[viperML]

I still don't see any perf impact apart from the few milliseconds that nvd takes:

$ time nixos-rebuild switch --fast --use-remote-sudo -f $D/hosts/zorah
________________________________________________________
Executed in   12.78 secs    fish           external
   usr time    7.90 secs  812.00 micros    7.90 secs
   sys time    1.02 secs  149.00 micros    1.02 secs	

$ time nh os switch -f $D/hosts/zorah
________________________________________________________
Executed in   13.27 secs    fish           external
   usr time    8.43 secs    1.02 millis    8.43 secs
   sys time    1.23 secs    0.00 millis    1.23 secs