We already had a featuretag for clientupdate, but the CLI wasn't using
it, making the "minbox" build (minimal combined tailscaled + CLI
build) larger than necessary.
Updates #12614
Change-Id: Idd7546c67dece7078f25b8f2ae9886f58d599002
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>