tool/gocross: retry downloading Go three times

Occasionally CI jobs will flake because downloading from GitHub fails.
Allow retrying up to 3 times to reduce CI flakiness.

Updates #cleanup

Change-Id: Ib019e89ac74b81d78f71a40099b20ff60014a81f
Signed-off-by: Alex Chan <alexc@tailscale.com>
This commit is contained in:
Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f