[MacPorts] #61435: use_xcode default value is incorrect on macOS 11.0

[MacPorts]

#61435: use_xcode default value is incorrect on macOS 11.0
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  Reporter:  nicst35  |      Owner:  (none)
      Type:  defect   |     Status:  new
  Priority:  Normal   |  Milestone:  MacPorts 2.6.4
 Component:  base     |    Version:  2.6.99
Resolution:           |   Keywords:  bigsur
      Port:           |
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Comment (by jmroot):

 The cache already existed on previous macOS versions. They just decided
 that since all dyld operations use the cache anyway, they might as well
 just pre-populate the cache with all system libs and remove the filesystem
 versions. Irritating for some use cases but it makes sense from a certain
 point of view.

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Ticket URL: <https://trac.macports.org/ticket/61435#comment:10>
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