Use a channel to update the check_all progress

The previous code was checking the threads in the order they were
created. So the progress update would be blocked on an earlier thread
even if later thread were already done.

Add to that that multiple instances of `cargo build` cannot run in
parallel, they will be serialized instead. So if the exercises needs to
be recompiled, depending on the order those `cargo build` are run,
the first update can be a long time coming.

So instead of relying on the thread terminating, use a channel to get
notified when an exercise check is done, regardless of the order they
finish in.

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