Change where plugins are created in the builddir
KCoreAddons >= 5.90 changed the behaviour of where kcoreaddons_add_plugin() puts the plugins .so files in the builddir, i.e. it doesn't use a plugins/ subdir any more. For details see [1].
Use /bin/k3b_plugins instead, as builddir/bin/k3b is going to be used for the app executable.