Consider fetching up to date mirror list before downloading artifacts

[Techatrix]

The community-mirrors.txt may change over time so when trying to build an old version of a flake, it may fetch from mirrors that are no longer in included in the current list of mirrors. It also won't pick up any new mirrors that have been added.

Wouldn't it be possible for fetch_builder.sh to first fetch the current version of community-mirrors.txt and then only fall back to the prefetched one if that failed?

[silversquirl]

good point! this should be pretty easy to implement - feel free to PR, otherwise I'll take a look when I have a spare moment ^-^