backupiso: Update to ISO file naming - bug#12932

backupiso: Update to ISO file naming - bug#12932

- commit https://git.ipfire.org/?p=ipfire-2.x.git;a=commit;h=fbd0608c2cb5372fff7857065ec7e605b1bf9cf7
   aligned the ISO file name to the image file name. This change also needed to be added
   to backupiso as the filename is used to download the iso from the IPFire server when
   creating an ISO backup.

Fixes: Bug#12932
Suggested-by: Matthias Fischer <matthias.fischer@ipfire.org>
Tested-by: Adolf Belka <adolf.belka@ipfire.org>
Signed-off-by: Adolf Belka <adolf.belka@ipfire.org>
Reviewed-by: Bernhard Bitsch <bbitsch@ipfire.org>