[MacPorts] #61429: Add macOS 11 Big Sur GitHub Actions

[MacPorts]

#61429: Add macOS 11 Big Sur GitHub Actions
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 Reporter:  ryandesign      |      Owner:  admin@…
     Type:  enhancement     |     Status:  new
 Priority:  Normal          |  Milestone:
Component:  server/hosting  |    Version:
 Keywords:                  |       Port:
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 GitHub Actions offers macOS 11 Big Sur as a preview:

 https://github.com/actions/virtual-environments/issues/1814

 macOS 11 final is expected to be released tomorrow November 12, 2020 so
 maybe GitHub Actions will update that to the final version soon.

 I am guessing the file to modify is browser:macports-
 ports/.github/workflows/main.yml

 I assume that we should be using Xcode 12.2 or later for that.

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Ticket URL: <https://trac.macports.org/ticket/61429>
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