fix precedence of CUDA versions in local 12.8 migrator

Author: h-vetinari

.ci_support/migrations/cuda129.yaml

@@ -16,8 +16,8 @@ __migrator:
     cuda_compiler_version:
       - 12.4
       - 12.6
-      - 12.8
       - None
+      - 12.8
       - 12.9
       - 13.0
       # to allow manual opt-in for CUDA 11.8, see