Add prefers tag to cuda_aware repositories

mpcdf_common.py

@@ -814,7 +814,8 @@ Macros:
                 if "openmpi" in mpi:
                     repo(cuda + "_aware_" + mpi + "_" + compiler,
                          ((project, cuda + "_" + mpi + "_" + compiler),),
-                         cuda_aware_mpi=True)
+                         cuda_aware_mpi=True,
+                         additional_tags=("Prefer: mpcdf_mpi_" + mpi + "_" + cuda,))
 
     if old_repos and not remove_old:
         print("Warning: Keeping the prjconf sections for the following obsolete repositories:")