refactor(3p/tvix): convert NixList usage to shared_ptr

Starting to reclaim memory. Fairly simple mechanical replacement.

Change-Id: I6b4c5c5596729470d1a049eba61e69e8097decf4
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1971
Tested-by: BuildkiteCI
Reviewed-by: glittershark <grfn@gws.fyi>

third_party/nix/src/libexpr/eval.hh

@@ -258,7 +258,7 @@ class EvalState {
   Value* allocAttr(Value& vAttrs, const Symbol& name);
 
   // Create a list value from the specified vector.
-  void mkList(Value& v, NixList* list);
+  void mkList(Value& v, std::shared_ptr<NixList> list);
 
   // Create a list value, allocating as many elements as specified in
   // size. This is used for the many cases in this codebase where