Since 473604f567 (CL/2910), depot hasn't
been directly exposing `pkgs` as an attribute, instead exposing it via
third_party.nixpkgs - the xanthous shell.nix isn't checked in CI, so it
was missed as part of that refactor - this updates it to be in line with
that
Change-Id: I12b081c16d53c6798f51ec6660ffa6d345870580
Reviewed-on: https://cl.tvl.fyi/c/depot/+/3176
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>
80d501d553
