Drop the llvm buildrequires back to rhel8

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

rust-toolset.yaml

@@ -10,7 +10,7 @@ data:
   dependencies:
     - buildrequires:
         platform: [el8]
-        llvm-toolset: [rhel_next]
+        llvm-toolset: [rhel8]
         rust-toolset: [rhel_next]
       requires:
         platform: [el8]