Why can't invent's CI build eventview's release branch?

[Glen Ditchfield]

Eventview's release/21.08 branch builds successfully in Jenkins
(https://build.kde.org/job/Applications/job/eventviews/), but not
in invent.kde.org.  The most recent attempt at the moment is
https://invent.kde.org/pim/eventviews/-/jobs/128298
which fails during compilation:  one error message is `expected class 
name`.  It seems to be objecting to `CalendarSupport::PluginFactory`, 
which puzzles me, because calendarsupport's release/21.08 plugin.h does 
declare `PluginFactory`.

Could invent be using a broken version of calendarsupport?