Fix go_1_6 test

In a user namespace, sending credentials for an unmapped user return
EINVAL instead of EPERM. So handle that case.

pkgs/development/compilers/go/1.6.nix

@@ -110,6 +110,7 @@ stdenv.mkDerivation rec {
 
   patches = [
     ./remove-tools-1.5.patch
+    ./creds-test.patch
   ];
 
   GOOS = if stdenv.isDarwin then "darwin" else "linux";