[PostGIS] #5566: Add CI testing under sandboxed system

[PostGIS]

#5566: Add CI testing under sandboxed system
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  Reporter:  strk          |      Owner:  robe
      Type:  task          |     Status:  new
  Priority:  medium        |  Milestone:  Website Management, Bots
 Component:  QA/buildbots  |    Version:
Resolution:                |   Keywords:  pgextwlist
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Comment (by strk):

 CI is failing due to our own script refusing to package functions owned by
 a different owner than the extension owner:
 https://woodie.osgeo.org/repos/30/pipeline/1441/4#L5492

 Should we relax that check and accept mixed ownership as long as the owner
 of existing function is a superuser ?
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Ticket URL: <https://trac.osgeo.org/postgis/ticket/5566#comment:7>
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