This will start including the sytray app in unstable builds for Linux,
unless the `ts_omit_systray` build flag is specified.
If we decide not to include it in the v1.88 release, we can pull it
back out or restrict it to unstable builds.
Updates #1708
Change-Id: Ia101a4a3005adb9118051b3416f5a64a4a45987d
Signed-off-by: Will Norris <will@tailscale.com>
Will Norris