refactor(3p/nix/libexpr): Use std::string as qualified type

Replaces most uses of `string` with `std::string`.

This came up because I removed the "types.hh" import from
"symbol-table.hh", which percolated through a bunch of files where
`string` was suddenly no longer defined ... *sigh*

third_party/nix/src/libexpr/common-eval-args.hh

@@ -19,6 +19,6 @@ struct MixEvalArgs : virtual Args {
   std::map<std::string, std::string> autoArgs;
 };
 
-Path lookupFileArg(EvalState& state, string s);
+Path lookupFileArg(EvalState& state, std::string s);
 
 }  // namespace nix