Support installing packages that are marked as insecure

[alexdavid]

Right now there is no way to depend on pkgs.rubyPackages_2_7.sass for example since this is marked as insecure in nixpkgs.

Here is a workaround that works for now, but is cumbersome:

garn.mkPackage(garn.nix.nixRaw`(import "\${nixpkgs}" { config.permittedInsecurePackages = ["ruby-2.7.8" "openssl-1.1.1w"]; inherit system; } ).rubyPackages_2_7.sass`)])