Include possible distprefix in --define dist for Forge-based packages
In forge based packages, and most notably all Golang packages, dist tag are
computed by a lua script in order to generate a snapshot date and shortcommit.
See
(https://src.fedoraproject.org/rpms/redhat-rpm-config/blob/master/f/forge.lua#_196).
By specifying dist in rpmbuild command line, computed dist tag is overridden and
as such produce an erroneous package name. For example, instead of
`golang-github-anacrolix-dms-0-0.1.20190304git8af4925.fc31` we get
`golang-github-anacrolix-dms-0-0.1.fc31`.
This patch proposes to pass the computed %{?distprefix} in dist to take into
account the specificity of Forge based packages.
Merges: https://pagure.io/fedpkg/pull-request/320
Signed-off-by: Robert-André Mauchin <zebob.m@gmail.com>