sched_ext: Make scx_group_set_weight() always update tg->scx.weight

Otherwise, tg->scx.weight can go out of sync while scx_cgroup is not enabled
and ops.cgroup_init() may be called with a stale weight value.

Signed-off-by: Tejun Heo <tj@kernel.org>
Fixes: 81951366 ("sched_ext: Add cgroup support")
Cc: stable@vger.kernel.org # v6.12+