In prep for a future port 80 MITM fix, make the 'debug ts2021' command
retry once after a failure to give it a chance to pick a new strategy.
Updates #13597
Change-Id: Icb7bad60cbf0dbec78097df4a00e9795757bc8e4
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>