fix: Simplify provisionerd job acquire (#158)

This uses a simple channel to detect whether a
job is running or not, and moves all cancels
to be in goroutines.

coderd/coderdtest/coderdtest.go

@@ -125,7 +125,7 @@ func New(t *testing.T) Server {
 	}
 
 	handler := coderd.New(&coderd.Options{
-		Logger:   slogtest.Make(t, nil),
+		Logger:   slogtest.Make(t, nil).Leveled(slog.LevelDebug),
 		Database: db,
 		Pubsub:   pubsub,
 	})