Split string to interpret "srun --label" correctly

[trhille]

Split string "srun --label" so that it is interpretted correctly when passed to check_call.

Checklist

• Document (in a comment titled Testing in this PR) any testing that was used to verify the changes

[trhille]

Testing

After updating to compass 2.0.0-alpha.1, I got this error when running the landice/antarctica/mesh_gen case:

Running: srun --label -n 128 ESMF_RegridWeightGen --source BedMachineAntarctica_2020-07-15_v02.scrip_masked.nc --destination Antarctica_scrip.nc -
-weight gridded_to_MPAS_weights.nc --method conserve --netcdf4 --dst_regional --src_regional --ignore_unmapped
      Failed
Exception raised while running the steps of the test case
...
FileNotFoundError: [Errno 2] No such file or directory: 'srun --label'

It seems that check_call() interpreted srun --label as a single string instead of instead of running srun with --label as a separate argument.

With these changes, the srun command is being interpreted correctly: the 8–40km antarctica case finished successfully in 34:24 , and a humboldt case ran successfully in 10.5 minutes.

Note: CI is currently failing for the Greenland mesh_gen test due to maintenance on LCRC server.