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.cc

@@ -48,7 +48,7 @@ Bindings* MixEvalArgs::getAutoArgs(EvalState& state) {
   return res;
 }
 
-Path lookupFileArg(EvalState& state, string s) {
+Path lookupFileArg(EvalState& state, std::string s) {
   if (isUri(s)) {
     CachedDownloadRequest request(s);
     request.unpack = true;