sched: Fix PROXY_EXEC sched_class->update_curr()

The point with PROXY_EXEC is that any and all runtime accounting goes to
the proxy rather than the current.

scheduler_tick() already calls into sched_class->task_tick() with the proxy
as parameter; unfortunately, sched_class->update_curr() doesn't take a task
as parameter and is incorrectly using rq->curr.

Change that to rq->proxy.

Note that this doesn't impact CFS which was using

  update_curr(cfs_rq_of(&rq->curr->se))

which will be the same as

  update_curr(cfs_rq_of(&rq->proxy->se))

Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>