dante: fix runtime on musl.

[ericonr]

musl implements the sched_{set,get}scheduler functions as stubs.
Unfortunately, dante fails if the functions are available but return an
error, which is always the case for musl. Force it to think setscheduler
isn't available (var appropriated from Alpine).

Fixes #28180

General

• This is a new package and it conforms to the quality requirements

Have the results of the proposed changes been tested?

• I use the packages affected by the proposed changes on a regular basis and confirm this PR works for me
• I generally don't use the affected packages but briefly tested this PR