Previously this used git checkouts of home-manager.git and nixpkgs.git on
whitby, which is self-evidently non-reproducible, so instead use pinned
tarballs of those repositories as data sources.
Change-Id: I3bd01e44d746c62e6bfbeab2e7698ae9954c8a94
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1993
Tested-by: BuildkiteCI
Reviewed-by: multi <depot@in-addr.xyz>
