mockgen: fix tests

pkgs/development/tools/mockgen/default.nix

···
-{ buildGoModule, lib, fetchFromGitHub }:
+{ buildGoModule, fetchFromGitHub, lib }:
+
 buildGoModule rec {
   pname = "mockgen";
   version = "1.6.0";
+
   src = fetchFromGitHub {
     owner = "golang";
     repo = "mock";
     rev = "v${version}";
     sha256 = "sha256-5Kp7oTmd8kqUN+rzm9cLqp9nb3jZdQyltGGQDiRSWcE=";
   };
+
   vendorSha256 = "sha256-5gkrn+OxbNN8J1lbgbxM8jACtKA7t07sbfJ7gVJWpJM=";
 
-  doCheck = false;
+  subPackages = [ "mockgen" ];
 
-  subPackages = [ "mockgen" ];
+  preCheck = ''
+    export GOROOT="$(go env GOROOT)"
+  '';
 
   meta = with lib; {
     description = "GoMock is a mocking framework for the Go programming language";