[rhendric]

The infra project currently references the infra branch, which holds commits that hard-code two behaviors: dropping store references to executable tools, and adding extra options to nixpkgs-review. This PR makes it possible for infra to reference the main branch instead, provided that the infra project does the following:

• Overrides the nixpkgs-update package with withStaticToolPaths = false;
• Adds

   --nixpkgs-review-opt 'extra-nixpkgs-config={ allowBroken = false; }' \
   --nixpkgs-review-opt num-eval-workers=3 \
   --nixpkgs-review-opt max-memory-size=4096
  

  to invocations of nixpkgs-update

When this merges, I'll provide the PR to infra to switch the reference and add these changes.

Closes #461.