Capture Nix stats during eval about ofborg

[vcunat]

Oh, right, even before merging :-) Timings might not be stable enough, but the counts should be sufficient to spot significant regressions.

[vcunat]

We have https://hydra.nixos.org/job/nixpkgs/trunk/metrics#tabs-charts

[grahamc]

Yeah, I'd like to say exactly how a PR changed nixpkgs's evaluation stats during evaluation time. This way we can spot big increases, and celebrate decreases. I suppose I could look at how Hydra captures this data.

[grahamc]

This all started, btw, because I had to bump the initial heap from the default to 4g the other day, and I'd like to catch future big jumps before ofborg is suddenly failing on master.