cmd/containerboot: use TS_AUTHKEY as the parameter for auth keys

We still accept the previous TS_AUTH_KEY for backwards compatibility, but the documented option name is the spelling we use everywhere else.

Updates #6321

Signed-off-by: David Anderson <danderson@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