fix(3p/nix): Use a proper pointer in Env to carry with-attrs

This eliminates the value-smuggling that would trip up the GC.

Change-Id: I8057df78cf0bf6bea9faf1b44233aa9820ae44f5
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1504
Tested-by: BuildkiteCI
Reviewed-by: glittershark <grfn@gws.fyi>
Reviewed-by: tazjin <mail@tazj.in>

third_party/nix/src/libexpr/eval.hh

@@ -42,6 +42,7 @@ struct Env : public gc {
   unsigned short prevWith : 14;  // nr of levels up to next `with' environment
   enum { Plain = 0, HasWithExpr, HasWithAttrs } type : 2;
   std::vector<Value*, traceable_allocator<Value*>> values;
+  Expr* withAttrsExpr = nullptr;
 };
 
 Value& mkString(Value& v, const std::string& s,