Update and fix nix flake (#341)

Since nix tries to update inputs on newer versions out of, I suppose,
some introduced incompatibility, `nix build` fails. And nixpkgs from
2023 sounds rather obsolete at this point, so update was kinda due I
think.
On newer versions of nixpkgs however buildGoModule has changed and
requires one of its arguments to be called vendorHash and not
vendorSha256, this patch changes that as well.

Fixes #297, closes #314, closes #307

nix/prefetcher.nix

@@ -9,7 +9,7 @@ pkgs.callPackage (import ./.) {
     ## `pkgs.buildGo121Module` in Nixpkgs.
     ## `buildGoModule` is set as `pkgs.buildGoModule` to try and work around
     ## `vendorHash` issues in the future.
-  vendorSha256 = sha256;
+  vendorHash = sha256;
 }
 // {
   outputHash = sha256;