Zims update script has been slightly mangled since nix-prefetch stopped
working.
Now it is updated to use nix-prefetch-url and no longer pulls from the
Torrent sources. That script exports a regular SHA256 hash and not an
SRI signature, so we now convert that to SRI as a second step in the
pre-fetch pipline
Also adding a cron to run the tool every month on the first, in order to
keep it up to date.
