[MacPorts] #63348: port @2.7.1 cleans when upgrading although asked via -sk not to do so

[MacPorts]

#63348: port @2.7.1 cleans when upgrading although asked via -sk not to do so
-----------------------+--------------------
 Reporter:  ballapete  |      Owner:  (none)
     Type:  defect     |     Status:  new
 Priority:  Normal     |  Milestone:
Component:  base       |    Version:  2.7.1
 Keywords:             |       Port:  port
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 `port -vd upgrade -sk imlib2` cleans in the `build` directory although it
 should `keep` it. Proved a few times on Big Sur, Version 11.5.

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Ticket URL: <https://trac.macports.org/ticket/63348>
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