[std/channels] change the order of signal and unlock

[ringabout]

Related #17392

The pthread_cond_broadcast() or pthread_cond_signal() functions may be called by a thread whether or not it currently owns the mutex that threads calling pthread_cond_wait() or pthread_cond_timedwait() have associated with the condition variable during their waits; however, if predictable scheduling behavior is required, then that mutex shall be locked by the thread calling pthread_cond_broadcast() or pthread_cond_signal().

Especially we need predictable scheduling behavior

if predictable scheduling behavior is required, then that mutex shall be locked by the thread calling pthread_cond_broadcast() or pthread_cond_signal().

[timotheecour]

@xflywind this PR makes sense and indeed improves #17392, see folowup notes in #17392 (comment)