with-transaction doesn't respect `nested-transaction-rule`

[qnkhuat]

this should raise an exception but currently we allow it

(t2/with-connection [conn]
  (t2/with-transaction [t-conn toucan2.connection/*current-connectable* {:nested-transaction-rule :prohibit}]
    (t2/with-transaction [t-conn2]
      (println t-conn2))))

sanity check for this behavior from next.jdbc

(t2/with-connection [conn]
 (binding [next.jdbc.transaction/*nested-tx* :prohibit]
  (next.jdbc/with-transaction [t-conn conn {}]
    (next.jdbc/with-transaction [t-conn conn {}]))))

;; => raise an exception with message "Nested transactions are prohibited"

[camsaul]

Fixed in 1.0.532