flush to eval cache

[tomberek]

Occasionally flush the eval cache transaction to allow easier checkpointing and restart.

[Mic92]

There is no eval cache in nix-eval-jobs. For CI usage it would be not very effective, except for rare rebuilds of the same commit. We could enable the cache as an opt-in for other use cases.

[Mic92]

It would be interesting to incorporate a more useful eval cache into nix-eval-jobs: https://github.com/numtide/nix-eval-cache (this one works more like make using timestamps and is more granular as it looks at files rather than git commit).