Fix regression for cdrom: sources from latest security update

Fix regression for cdrom: sources from latest security update

Skip a reverify for cdrom: sources. The reverify step is actually
harmful here because the apt-cdrom add code uses the indexcopy.cc
which will "normalize" the Packages file from the cdrom when it
writes it to the local disk. This leads to changing the "MD5sum"
field (notice the lower case "s") on the cdrom Packages file to
a "MD5Sum" field on the local file in /var/lib/apt/lists. Which
of course alters the hash and makes apt fail to reverify the file.