Ensure that archive members are added in sorted order with a fixed
mtime. This allows `nix-build --check` to succeed (when building a
tarball of a simple system configuration).
We also remove env-vars which doesn't appear to do much apart from
capture a bunch of store paths we probably don't want.
This is an alternative to
https://github.com/NixOS/nixpkgs/pull/2281/commits/4b78a5b5fb8584347999832b3c67d1276488140b