Error retreving chem input file

[jedwards4b]

@nanr and I noticed this in cesm2.1 recently the aircraft_specifier for ac_CO2 refers to a text file: atm/cam/ggas/emissions-cmip6_CO2_anthro_ac_ScenarioMIP_IAMC-AIM-ssp370ext_209901-250112_fv_0.9x1.25_c20211217.txt and within that file a netcdf file of the same name but with .nc extension is referenced.

The problem is on remote systems where inputdata may not be populated with these files. Only the text file is listed in BuildConf/cam.input_data_list and so the netcdf file is not downloaded, resulting in a runtime error.