Fix channel label format in mpi-hpc.yml

Author: heather999

conda/lock/mpi-hpc.yml

@@ -1,6 +1,6 @@
 # mpi-hpc.yml
 channels:
-  - conda-forge/label/mpi_external
+  - conda-forge/label/mpi-external
   - conda-forge
 dependencies:
   - mpich