[MacPorts] #67124: xar: Use github.tarball_from archive

[MacPorts]

#67124: xar: Use github.tarball_from archive
------------------------+-------------------------
 Reporter:  ryandesign  |      Owner:  Schamschula
     Type:  defect      |     Status:  assigned
 Priority:  Normal      |  Milestone:
Component:  ports       |    Version:
 Keywords:              |       Port:  xar
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 The xar port uses the github portgroup and does not specify a value for
 `github.tarball_from`, therefore it uses the default value, `tarball`,
 which is deprecated. ''When you update the port to the next version'',
 please add the line `github.tarball_from archive` to the Portfile. (Adding
 this line ''before'' updating to the next version will cause checksum
 mismatches.)

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Ticket URL: <https://trac.macports.org/ticket/67124>
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