top-level: fix nix-shell eval w/nixUnstable

[Ma27]

Motivation for this change

For a full description of the underlying issue please read
NixOS/nix#4003

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

[Ma27]

Note: IMHO we should backport this to 20.09 and 20.03 to allow users of nixUnstable to use nix-shell on their nixpkgs checkouts.

[Ma27]

Backported to 20.03 as 732684b, to 20.09 as c1dbaed.

[nh2]

I just upgraded to nix 2.4 stable and found this issue when trying to reproduce a nixpkgs 20.03 build (from before the backport patch).

It is quite unfortunate that with nix >= 2.4, you cannot reproduce old builds at all now, because they don't evaluate, breaking the reproducibility story.

Is there something that could be done in nix to make that possible?

[andir]

I'm trying to fix that over here but that isn't as straight forward.