Fix `package_cache_async` missing from `ResolvedContext` dict round-trip

[nrusch]

This is a follow-up fix to https://github.com/AcademySoftwareFoundation/rez/pull/1679

The new package_cache_async attribute on the ResolvedContext class was not being round-tripped through the context serialization process. This leads to the following error after spawning a shell (when the rezolve context command runs):

17:44:39 ERROR    ResolvedContextError: Failed to load context from S:\Temp\rez_context_gsab2kwu\context.rxt: AttributeError: 'ResolvedContext' object has no attribute 'package_cache_async'

I will note that it seems a bit strange for ResolvedContext.from_dict to call context._update_package_cache() after creating a new instance, but I assume there are some historical reasons for this. However, given this pattern, if package_cache_async is set to False, from_dict could potentially block for a very long time.

CC @isohedronpipeline for visibility.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 58.52%. Comparing base (3d0f224) to head (fd541f5). Report is 37 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1810   +/-   ##
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  Coverage   58.52%   58.52%           
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  Files         126      126           
  Lines       17206    17207    +1     
  Branches     3519     3519           
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+ Hits        10069    10070    +1     
  Misses       6468     6468           
  Partials      669      669           

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