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/exec.go

@@ -40,9 +40,6 @@ type ExecPath struct {
 	W bool
 }
 
-// SetSchedIdle is whether to set [ext.SCHED_IDLE] scheduling priority.
-var SetSchedIdle bool
-
 // GetArtifactFunc is the function signature of [FContext.GetArtifact].
 type GetArtifactFunc func(Artifact) (*check.Absolute, unique.Handle[Checksum])
 
@@ -427,7 +424,6 @@ func (a *execArtifact) makeContainer(
 	z.ParentPerm = 0700
 	z.HostNet = hostNet
 	z.Hostname = "cure"
-	z.SetScheduler = SetSchedIdle
 	z.SchedPolicy = ext.SCHED_IDLE
 	if z.HostNet {
 		z.Hostname = "cure-net"
@@ -609,6 +605,7 @@ func (a *execArtifact) cure(f *FContext, hostNet bool) (err error) {
 	); err != nil {
 		return
 	}
+	z.SetScheduler = f.cache.flags&CSchedIdle != 0
 
 	var status io.Writer
 	if status, err = f.GetStatusWriter(); err != nil {