USDZ conversion currently requires to change the 'current working
directory', due to internal USD library limitations.
This can only be done from main thread (and is highly discouraged in
general). This implies that USDZ conversion cannot happen from the Job
worker thread.
This commit moves it to the `endjob` callback of the USD export job
instead, as it is guaranteed to be executed from caller thread, i.e.
main thread in this case.
Bastien Montagne