fix: Correct DebCowbuilder handling for existing images

In pull request #482 introducing the DebCowbuilder class, there is a
wrong assumption being made on the type of the file system entry holding
the chroot. While it is correct for the base pbuilder case that this
entry have to be a file, the cowbuilder case handles directories. But
the base class used by DebCowbuilder only checks for regular file.
Hence, it triggers an error in twistd.log and makes the step failing
after a first run.