fix: Fixes nix install by using correct pkg-configDepends config key

[TimDeve]

As reported by Hrafn Blóðbók, nix updated references to pkgconfig in its codebase to use the pkg-config spelling, this seems to break the installation of Carp using nix.

See this PR: NixOS/nixpkgs@9bb3fcc

The current error when installing:

error: anonymous function at /nix/var/nix/profiles/per-user/root/channels/nixos/pkgs/development/haskell-modules/generic-builder.nix:13:1 called with unexpected argument 'pkgconfigDepends'

       at /nix/var/nix/profiles/per-user/root/channels/nixos/lib/customisation.nix:69:16:

           68|     let
           69|       result = f origArgs;
             |                ^
           70|

We might need to update the nix action as well, just pushing it like this for now.

[TimDeve]

Updated the Github Actions so it uses 21.05, guess it means this is now the minimum Nix version to install Carp with, hope that doesn't break anyone's workflows.