CPack/DEB: fix `md5sums` paths in dbgsym packages

The Debian `lintian` tool reported the dbgsym package issue as
`md5sums-lists-nonexistent-file`.

The DEB generator writes `md5sums` entries by stripping a top-level prefix
from each packaged file path. That used `CPACK_TEMPORARY_DIRECTORY`, which
works for regular debs but not for dbgsym packages.

Dbgsym package files are collected from `GEN_DBGSYMDIR` and passed to
`DebGenerator` with `WorkDir` rooted there. Because the `md5sums` writer
stripped `TemporaryDir` instead, the prefix did not match and absolute
staging paths leaked into `md5sums`.

Strip `WorkDir` instead. That is the actual root of `PackageFiles` for both
regular deb and dbgsym package generation, so `md5sums` entries stay
relative in both cases.