calendar-server: Fix double-free detection abort

app_notify_events_added uses an intermediate builder to construct an
array that is then added to the main variant using g_variant_builder_add
which should clear the intermediate, but doesn't due to the way it is
passed: by value, rather than as a pointer.

This was debugged with the help of Eduardo Habkost, who believes it
works on x86 due to big structs being passed as pointers.

Fixed: https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/3440
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1848>


(cherry picked from commit 404ca919)