hydra-eval-jobs: Ugly hackery to reduce memory usage

You can now set 'evaluator_max_heap_size' to make hydra-eval-jobs restart itself if the Boehm heap exceeds the specified size.

For example, with 'evaluator_max_heap_size = 256000000',

$ hydra-eval-jobs '<nixpkgs/pkgs/top-level/release.nix>' -I nixpkgs=channel:nixos-17.09

has a max RSS of .56 GiB rather than 4.7 GiB.

Unfortunately it doesn't help much for the NixOS jobsets because of the "tested" job which requires a huge amount of memory all by itself.

Created by  Eelco Dolstra  on June 5, 2018
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