`env` referred from top-level callinfo should not be unshared; fix #4019

Because `mirb` keeps top-level environment across each iteration.

src/vm.c

@@ -300,6 +300,7 @@ mrb_env_unshare(mrb_state *mrb, struct REnv *e)
 
     if (!MRB_ENV_STACK_SHARED_P(e)) return;
     if (e->cxt != mrb->c) return;
+    if (e == mrb->c->cibase->env) return; /* for mirb */
     p = (mrb_value *)mrb_malloc(mrb, sizeof(mrb_value)*len);
     if (len > 0) {
       stack_copy(p, e->stack, len);