fix(3p/nix/libexpr): Make EvalState internals visible to GC

Change-Id: Ia7de49acb859040429fdd8ab143d485589ac02d4
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1277
Reviewed-by: glittershark <grfn@gws.fyi>
Reviewed-by: isomer <isomer@tvl.fyi>
Reviewed-by: Kane York <rikingcoding@gmail.com>
Tested-by: BuildkiteCI

third_party/nix/src/libexpr/eval.hh

@@ -65,7 +65,7 @@ typedef std::map<Path, Expr*, std::less<Path>,
                  traceable_allocator<std::pair<const Path, Expr*>>>
     FileParseCache;
 
-class EvalState {
+class EvalState : public gc {
  public:
   SymbolTable symbols;