applets/digital-clock: Fix broken binding in the list of calendar plugins

First rule of the fighting club: never bind to a list. You never know
what the heck you're up against.

On the other hand, assigning FormData.buddyFor in a procedural way in
the Repeater's hook is robust, and there's nothing that could go wrong.

CCBUG: 468712

See also: plasma-desktop!1490

(cherry picked from commit a72ca3ac)