Fix#136 (stop pulling the files from @griff's poor server), also add a script
to update the files.
The fun thing about this is that due to sourcing the files from
`https://gitlab.com/simple-nixos-mailserver/nixos-mailserver/raw/master/tests/clamav`
during the tests, updates to the `hashes.json` and `*.cvd` files will always
fail CI. I guess this is a reasonable tradeoff as long as people are aware of
it.
