Var: Fix cacheable issue for njs variable access

The variables accessed with JS template literal should be not cacheable.
Since it is parsed by njs engine, Unit can't create indexes on these
variables for caching purpose. For example:

   {
       "format": "`{bodyLength:\"${vars.body_bytes_sent}\",status:\"${vars.status}\"}\n`"
   }

The variables like the above are not cacheable.

Closes: #1169

src/nxt_var.c

@@ -147,7 +147,7 @@ nxt_var_ref_get(nxt_tstr_state_t *state, nxt_str_t *name, nxt_mp_t *mp)
 
     if (decl != NULL) {
         ref->handler = decl->handler;
-        ref->cacheable = decl->cacheable;
+        ref->cacheable = (mp == state->pool) ? decl->cacheable : 0;
 
         goto done;
     }