ci: reverse proxy for LAVA downloads

Enable a reverse proxy in between the google LAVA dispatcher and freedesktop.org, and document how to set it up. This is not a big deal right now when we only have 1 db410c job enabled in our pipelines, but will be a large reduction in artifacts download costs once we switch cheza over to LAVA.

@daniels @tomeu @chema @rellla @itoral for folks working on LAVA labs that probably want to enable this too.