This makes magrathea use the repostory version of nix and git. This is
done in the pursuit of enforcing guaranteed unified experience of
magrathea tool across all users of a `tvl-kit`-based
repository. Especially among ubuntu users with uncontrolled set of
packages and versions installed on their system.
Not having this was giving build problems for `mg build` as one of the
users has 2.17 version of nix that had inconsistent hash computation
with 2.3.
Change-Id: I3182faf4c545ac61f6cc1cc862dc23d51c1cd397
Reviewed-on: https://cl.tvl.fyi/c/depot/+/9892
Tested-by: BuildkiteCI
Reviewed-by: tazjin <tazjin@tvl.su>
e3fe6826f8