check_updates.py and file:// protocol

Hi, I've been experimenting a bit with slpkg; adding a local binary repo with 'MIRROR="file://..."', initial 'slpkg update' fails, but that can be fixed setting 'DOWNLOADER="curl"' in slpkg.toml.

However, subsequent 'slpkg update' fail because, as far as I can see, check_updates.py just doesn't support the 'file://' protocol.

It's not a big issues, as one can force the update, but I thought it'd be better to report this anyway.

Thanks