deploy: Fix mutex locking for global sync timeout

deploy: Fix mutex locking for global sync timeout

The locking here was always too long - by holding the mutex
during the `sync()` call, it means `g_cond_wait_until()` can
never wake up (because its API requires the mutex to be locked).

Confusingly though of course we do still print the "timed out"
message, and I think that tricked us when we were doing testing
here.

We only need to lock the mutex when we're manipulating shared
state, which basically boils down to the `gboolean success`.