Move vCallFlake into EvalState

This fixes a use-after-free bug:

1. s = new EvalState();
2. callFlake()
3. static vCallFlake now references s
4. delete s;
5. s2 = new EvalState();
6. callFlake()
7. static vCallFlake still references s
8. crash

Nix 2.3 did not have a problem with recreating EvalState.

src/libexpr/eval.hh

@@ -100,6 +100,7 @@ public:
     /* Store used to build stuff. */
     const ref<Store> buildStore;
 
+    RootValue vCallFlake = nullptr;
 
 private:
     SrcToStore srcToStore;