core/exec-invoke: call setpriority() after sched_setattr()

core/exec-invoke: call setpriority() after sched_setattr()

The nice value is part of struct sched_attr, and consequently invoking
sched_setattr() after setpriority() would clobber the nice value with
the default (as we are not setting it in struct sched_attr).

It would be best to combine both calls, but for now simply invoke
setpriority() after sched_setattr() to make sure Nice= remains effective
when used together with CPUSchedulingPolicy=.

(cherry picked from commit 711a157738b3dcd29a5ebc8f498eb46bfac59652)