Check nvr before build
When building for 'epel8', it is also triggered 'epel8-playground'
build. nvr have to be checked whether the build is already built.
There was the check, but it queried different nvr. Now build will
be terminated when there is the same nvr present in Koji.
Fixes: #356
JIRA: COMPOSE-3875
Signed-off-by: Ondrej Nosek <onosek@redhat.com>