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 <builddir>/bin/k3b_plugins instead, as builddir/bin/k3b is going to be
used for the app executable.

[1] frameworks/kcoreaddons@375e19cf