Rename pkgconfig to pkg-config

[svelterust]

Fixes following issue:

         at /nix/store/g68f5abh3xhcz8xsdlfw7wkgkkcx3nwy-source/pkgs/stdenv/genern.nix:347:7:

          346|       depsHostHost                = lib.elemAt (lib.elemAt depend          347|       buildInputs                 = lib.elemAt (lib.elemAt depend             |       ^
          348|       depsTargetTarget            = lib.elemAt (lib.elemAt depend
       (stack trace truncated; use '--show-trace' to show the full trace)

       error: 'pkgconfig' has been renamed to/replaced by 'pkg-config'

[mirrexagon]

Thanks!