Confine cleanup and use held session locks

internal/artifacts/lock_unsupported.go

@@ -0,0 +1,15 @@
+//go:build !linux && !darwin && !windows
+
+package artifacts
+
+import (
+	"fmt"
+	"os"
+	"runtime"
+)
+
+func acquireHeldFileLock(_ *os.File) error {
+	return fmt.Errorf("held file locks are unsupported on %s", runtime.GOOS)
+}
+
+func releaseHeldFileLock(_ *os.File) error { return nil }