nix: expose dapptoolsOverrides

[asymmetric]

So that overriding the dapptools version doesn't require overriding the whole nixpkgs-pin import.

So instead of:

❯ nix-shell --arg pkgs '(import (fetchGit { url = "https://github.com/makerdao/nixpkgs-pin"; rev = "eec6c4b433c02d02352ace97f9ae30ab3bbf9bd7"; ref = "master"; }) { dapptoolsOverrides = { current = fetchTarball https://github.com/dapphub/dapptools/archive/master.tar.gz; }; }).pkgs'

one can simply do:

❯ nix-shell --arg dapptoolsOverrides "{ current = fetchTarball https://github.com/dapphub/dapptools/archive/master.tar.gz; }"

[asymmetric]

Supersedes https://github.com/makerdao/nixpkgs-pin/pull/2.

[asymmetric]

PTALA.