It now strictly evaluates all remaining attributes, preventing
unevaluated thunks that cannot be garbage-collected. It's also applied
to all jobs in Nixpkgs' release.nix.
This reduces hydra-eval-jobs' memory consumption on the 14.12
release-combined jobset from 5.1 GB to 2.0 GB.
Original commit was reverted due to using features from a newer nix than
hydra had, hydra has since been updated
This reverts commit 07e726c85c8f36d3a27fb7254f09ac69c7b94c65.