Until this commit we had a single hash of the whole version response from
omahaproxy. This worked well for not updating unnecessarily but only until one
single channel has a new version available.
Unfortunately the dev version is released quite frequently, so the hash is of no
use at all (we could rather directly fetch everything everytime we execute the
script).
This led to this commit, which adds two methods of caching:
First of all, if a perticular version/channel is already in the previous version
of the sources.nix file, don't download it again.
And the second method is to check if the current sha256 is already downloaded
and reads the corresponding sha256 from the lookup table.
So, this should really help to avoid flooding the download servers and to not
stress impatient users too much.
6968400a34