ops.nixos.nixosFor is intended to provide the "basic" readTree-like
system arguments to NixOS systems; in particular, it provides "depot" as
a module argument, as well as, for the moment, config.depot.
Change-Id: I442c7d79ac0eb2ff8e1bf606f4e083e15eb0a8f4
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2761
Tested-by: BuildkiteCI
Reviewed-by: glittershark <grfn@gws.fyi>