spindle/engines/nixery: add workflow id to container labels

Adding the repo name there would've been useful but it's not readily
available within SetupWorkflow. We'll... figure out how to add that
later.

Signed-off-by: Anirudh Oppiliappan <[e-mail]>

spindle/engines/nixery/engine.go

···
 		Tty:        false,
 		Hostname:   "spindle",
 		WorkingDir: workspaceDir,
+		Labels: map[string]string{
+			"sh.tangled.pipeline/workflow_id": wid.String(),
+		},
 		// TODO(winter): investigate whether environment variables passed here
 		// get propagated to ContainerExec processes
 	}, &container.HostConfig{