look for SHA256SUMS instead of MD5SUMS, which is not published anymore

the code still calls everything "md5" but it's not md5 anymore, that
should be updated everywhere.

Signed-off-by: Mattia Rizzolo <mapreri@ubuntu.com>
1 job for dev-shadmod-231009-2225 in 0 seconds
Status Job ID Name Coverage
  External
failed managetests

00:00:08