`available_diags.000000` does not recognise the use of some diags with their CMOR names

[dougiesquire]

@anton-seaice noticed here that available_diags.000000 has:

"total_evap"  [Unused]
    ! modules: ocean_model
    ! dimensions: scalar
    ! long_name: Area integrated evap/condense at ocean surface
    ! units: kg s-1
    ! standard_name: water_evaporation_flux_area_integrated
    ! variants: {total_evap,total_evs}

despite the diagnostic being used via its CMOR name total_evs.

Possibly it is due to the use of register_scalar_field for this scalar field (vs register_diag_field for nd fields), but I need to look more closely.