cmd/tailscale/cli: clean up how optional commands get registered

Both @agottardo and I tripped over this today.

Updates #cleanup

Change-Id: I64380a03bfc952b9887b1512dbcadf26499ff1cd
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
This commit is contained in:
Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f