cmd/tailscale: omit the `drive` subcommand in MacOS GUI apps

In MacOS GUI apps, users have to select folders to share via the GUI. This is both because
the GUI app keeps its own record of shares, and because the sandboxed version of the GUI
app needs to gain access to the shared folders by having the user pick them in a file
selector.

The new build tag `ts_mac_gui` allows the MacOS GUI app build to signal that this
is a MacOS GUI app, which causes the `drive` subcommand to be omitted so that people
do not mistakenly attempt to use it.

Updates tailscale/tailscale#17210

Signed-off-by: Percy Wegmann <percy@tailscale.com>

cmd/tailscale/cli/drive.go

@@ -1,7 +1,7 @@
 // Copyright (c) Tailscale Inc & AUTHORS
 // SPDX-License-Identifier: BSD-3-Clause
 
-//go:build !ts_omit_drive
+//go:build !ts_omit_drive && !ts_mac_gui
 
 package cli