Fast-path in Channel prevents ProduceCoroutine from cancelling

[qwwdfsad]

After #2384 the following code now hangs instead of throwing CancellationException

@Test
fun p2384() = runBlocking {
    val myFlow = callbackFlow<Any> {
        close()

      // Should throw?
        delay(Duration.INFINITE)
    }

    // Should complete?
    myFlow.collect()
}