internal/pkg: exclusive artifacts

This alleviates scheduler overhead when curing many artifacts.

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

internal/rosa/busybox.go

@@ -25,6 +25,9 @@ func (a busyboxBin) Kind() pkg.Kind { return kindBusyboxBin }
 // Params is a noop.
 func (a busyboxBin) Params(*pkg.IContext) {}
 
+// IsExclusive returns false: Cure performs a trivial filesystem write.
+func (busyboxBin) IsExclusive() bool { return false }
+
 // Dependencies returns the underlying busybox [pkg.File].
 func (a busyboxBin) Dependencies() []pkg.Artifact {
 	return []pkg.Artifact{a.bin}
@@ -80,7 +83,8 @@ func newBusyboxBin() pkg.Artifact {
 		checksum = "L7OBIsPu9enNHn7FqpBT1kOg_mCLNmetSeNMA3i4Y60Z5jTgnlX3qX3zcQtLx5AB"
 	)
 	return pkg.NewExec(
-		"busybox-bin-"+version, nil, pkg.ExecTimeoutMax, fhs.AbsRoot, []string{
+		"busybox-bin-"+version, nil, pkg.ExecTimeoutMax, false,
+		fhs.AbsRoot, []string{
 			"PATH=/system/bin",
 		},
 		AbsSystem.Append("bin", "busybox"),
@@ -111,7 +115,7 @@ func (t Toolchain) newBusybox() pkg.Artifact {
 		env = append(env, "EXTRA_LDFLAGS=-static")
 	}
 
-	return t.New("busybox-"+version, stage3Concat(t, []pkg.Artifact{},
+	return t.New("busybox-"+version, false, stage3Concat(t, []pkg.Artifact{},
 		t.Load(Make),
 		t.Load(KernelHeaders),
 	), nil, slices.Concat([]string{