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.
This channel only builds a small subset of Nixpkgs, mostly suitable
for servers. Since the channel update doesn't require thousands of
packages to be built first, it should provide much faster turnaround
in case of security updates.
(cherry picked from commit 2c7acc6731ad4b05b1bbc3e632cdfcad4560ac22)
