MBS generates conflicts for the ursine stream rpms if a different stream
is used in buildrequires, but that breaks when they reuse the same rpms.
Resolves: RHEL-61966
Related: RHEL-61979
We can't build with a buildrequires on llvm rhel_next, because we also
need our previous rust build that requires llvm rhel8. So we'll rebuild
with rhel8 and a runtime requires on rhel_next (using llvm-compat), and
then a followup rebuild can be pure rhel_next.
Related: RHEL-40066
