feature/featuretags: make QR codes modular (#18358)

QR codes are used by `tailscale up --qr` to provide an easy way to
open a web-page without transcribing a difficult URI. However, there’s
no need for this feature if the client will never be called
interactively. So this PR adds the `ts_omit_qrcodes` build tag.

Updates #18182

Signed-off-by: Simon Law <sfllaw@tailscale.com>

util/qrcodes/qrcodes_linux.go

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