Revert "Make hydra-evaluator less likely to barf with a GC error"

This reverts commit c6a433439f035971ae1d60663755c544aacf6032. This makes *every* Nix child process reserve 12 GiB, even calls to nix-hash. So we get Sep 07 16:01:06 chef hydra-evaluator[18344]: GC Warning: Failed to expand heap by 12884901888 bytes Sep 07 16:01:06 chef hydra-evaluator[18344]: Can't start up: not enough memory Sep 07 16:01:06 chef hydra-evaluator[18344]: command `nix-prefetch-git /var/lib/hydra/scm/git/ad8945e296a15d6e5136ad2daee8742ae425ad26c467579cc280cdba649ffc51 5d9d07bbb1dbadd9d445b093e3a0d8247119860e' failed with exit status 256 at /nix/store/v3gdlj85i06g8qm3n0vmq8a1rci2myqn-hydra-0.1.1234.abcdef/libexec/hydra/lib/Hydra/Helper/Nix.pm line 425.