Integration with nix-fast-build

[lovesegfault]

Since reviews often involve building a huge number of packages, it'd be nice if there was an easy way to ask nixpkgs-review to use https://github.com/Mic92/nix-fast-build under the hood.

[lovesegfault]

One particularly useful outcome of this would be that it could support --retries.

[Mic92]

Just now we don't support non-flake usage in nix-fast-build, so it would be a bit slower to start nix-fast-build on large repos such as nixpkgs. Also I just recently benchmarked nix-eval-jobs on nixpkgs and it seems to be significantly slower than just using nix-env -> Have to check where this slow down is coming from.