fix: nix-shell: update `pkgconfig` -> `pkg-config` build input

[christoph-heiss]

Description

The (formerly) pkgconfig package in NixOS/nixpkgs has long been renamed to pkg-config, to match its upstream name. As of last september, this actually throws a hard error and thus breaks shell.nix. (See here).

Testing

Run nix-shell on a (recent) NixOS system or any system with the Nix package manager installed and a recent nixpkgs channel.

Issue(s)

Closes N/A.

Thanks for considering!

[jrconlin]

Thanks!