flake: fix eval warning

In change nkzyltot (2d2056ea, flake: fix eval warning), pkgs.system
was changed to pkgs.stdenv.hostPlatform.system. This was required after
https://github.com/NixOS/nixpkgs/pull/456527 made using pkgs.system
a warning

Unfortunately you missed one... oops

Signed-off-by: Skyler Grey <[e-mail]>

flake.nix

···
           };
         buildGoApplication =
           (self.callPackage "${gomod2nix}/builder" {
-            gomod2nix = gomod2nix.legacyPackages.${pkgs.system}.gomod2nix;
+            gomod2nix = gomod2nix.legacyPackages.${pkgs.stdenv.hostPlatform.system}.gomod2nix;
           }).buildGoApplication;
         modules = ./nix/gomod2nix.toml;
         sqlite-lib = self.callPackage ./nix/pkgs/sqlite-lib.nix {