Fixes the evaluation of an attrs using __toString with builtins.toJSON:

Before:

nix-repl> builtins.toJSON { __toString = _: "foo"; }
error: cannot convert a function to JSON

Now:

nix-repl> builtins.toJSON { __toString = _: "foo"; }
"\"foo\""

I tried to abstract this away a bit to not duplicate the logic.

Also I'm not happy about the noPos in the toJSON case, but I couldn't figure out how to get a correct Pos and I think that shouldn't be shown anyway.

cc @Ericson2314 who helped me find the error