This also rewrites a lot of the command handling in the fixtures
library, since we want to more precisely control which way that the nix
store is set up in the tests, rather than the previous method of
renaming /nix/store to some temp dir (which allows builds but does not
allow any /nix/store paths or stability across runs, which is a
significant issue for snapshot testing).
It uses a builder to reduce the amount of state carelessly thrown
around.
The evil NARs are inspired by CVE-2024-45593
(https://github.com/NixOS/nix/security/advisories/GHSA-h4vv-h3jq-v493).
No bugs were found in this endeavor.
Change-Id: Iee41b055fa96529c5a3c761f680ed1d0667ba5da
