Allow setting GC_INITIAL_HEAP_SIZE for hydra-eval-jobs

This cannot be done in the hydra-evaluator systemd unit, since then every other Nix process (e.g. hydra-evaluator and nix-prefetch-*) will also allocate the specified heap size, probably leading to OOM.

Created by  Eelco Dolstra  on May 16, 2018
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