Let `misc.disttag_re` match substrings
    
    Previously, the regular expression was anchored to the start of a
    string. This allows matching the dist tag e.g. inside a release string,
    which is needed for tagging only the latest builds per dist in
    `tag_package()`.
    
    Signed-off-by: Nils Philippsen <nils@redhat.com>