internal/pkg: per-cache SCHED_IDLE

This is cleaner than setting it globally, and is impossible to race.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/pkg/pkg.go

@@ -503,6 +503,10 @@ const (
 	// This behaviour significantly reduces performance and is not recommended
 	// outside of testing a custom [Artifact] implementation.
 	CValidateKnown = 1 << iota
+
+	// CSchedIdle arranges for the [ext.SCHED_IDLE] scheduling priority to be
+	// set for [KindExec] and [KindExecNet] containers.
+	CSchedIdle
 )
 
 // Cache is a support layer that implementations of [Artifact] can use to store