Add hint about downloadable logs to CI README

Hi,

On 2022-09-26 22:08:12 -0400, James Coleman wrote:

I was wondering about how to debug failed builds on Cirrus CI, and
after poking at the interface I realized we helpfully upload the logs
from CI runs for user download.

In an effort to save the next person a few minutes I thought the
attached minor patch would help.

I'm not quite sure how likely it is to help, but it can't hurt... Pushed.

Thanks for the patch!

Greetings,

Andres Freund