switch PATH_cache_finished to Threads.Condition

[ericphanson]

fixes #52890

Seems to work locally for me, though I'm not sure if this is the right fix

[IanButterworth]

This should be the fix #52893

[IanButterworth]

I'm going to close this as superseded, and I think tests are just going to hang

[ericphanson]

They ran ok locally fwiw

[IanButterworth]

Huh.. how did both tasks acquire the lock?

[ericphanson]

wait releases it:

help?> Threads.Condition
  Threads.Condition([lock])

  A thread-safe version of Base.Condition.

  To call wait or notify on a Threads.Condition, you must first call lock on it. When wait is called, the lock is atomically released during
  blocking, and will be reacquired before wait returns. Therefore idiomatic use of a Threads.Condition c looks like the following:

[IanButterworth]

My bad. Ok I'll update my PR. There are other fixes to make too based on Jameson's review