flake : use pkg-config instead of pkgconfig (#3188)

pkgconfig is an alias, it got removed from nixpkgs:
https://github.com/NixOS/nixpkgs/blob/295a5e1e2bacd6e246db8b2bb35d2a9415883224/pkgs/top-level/aliases.nix#L1408

flake.nix

@@ -34,7 +34,7 @@
             with pkgs; [ openblas ]
         );
         pkgs = import nixpkgs { inherit system; };
-        nativeBuildInputs = with pkgs; [ cmake ninja pkgconfig ];
+        nativeBuildInputs = with pkgs; [ cmake ninja pkg-config ];
         llama-python =
           pkgs.python3.withPackages (ps: with ps; [ numpy sentencepiece ]);
         postPatch = ''