cmd/tailscale/cli: set Sparkle auto-update on macsys (#9952)

On `tailscale set --auto-update`, set the Sparkle plist option for it.
Also make macsys report not supporting auto-updates over c2n, since they
will be triggered by Sparkle locally.

Updates #755

Signed-off-by: Andrew Lytvynov <awly@tailscale.com>

clientupdate/clientupdate.go

@@ -202,7 +202,9 @@ func (up *Updater) getUpdateFunction() (fn updateFunction, canAutoUpdate bool) {
 			// support auto-updates.
 			return up.updateMacAppStore, false
 		case version.IsMacSysExt():
-			return up.updateMacSys, true
+			// Macsys update func kicks off Sparkle. Auto-updates are done by
+			// Sparkle.
+			return up.updateMacSys, false
 		default:
 			return nil, false
 		}