frontend: add f21 chroot

Signed-off-by: Igor Gnatenko <i.gnatenko.brain@gmail.com>
(cherry picked from commit 3fa58dcd8c7ec0b8b0caeb9d91ca03bc8f263bb3)
Automatic commit of package [copr-frontend] release [1.39-1].
Automatic commit of package [copr-backend] release [1.37-1].