cmd/cigocacher: make token optional for --stats (#20896)

In cases where cigocacher is using a proxy that handles the auth, it's
possible to fetch stats without knowing our own access token. Don't fail
early if the access token isn't passed. If there's no proxy, it will
fail with the error from the gocached server.

Updates tailscale/corp#45427

Signed-off-by: Tom Proctor <tomhjp@users.noreply.github.com>