add timeout on steps with network I/O

Set a reasonable timeout to prevent jobs from hanging for hours (#50).
5 minutes is way longer than should ever be necessary, but accounts for
intermittent networking issues on either the actions runner or the
Tailscale package or control server.  Users can always set a shorter
timeout on their own action step.

Fixes #50

Signed-off-by: Will Norris <will@tailscale.com>