Odd fail when input data server and DIN_LOC_ROOT are the same.

While our main input data server was down for a week, we quickly made the NERSC DIN_LOC_ROOT (/global/cfs/cdirs/e3sm/inputdata) a server since it is accessible via the web at https://portal.nersc.gov/project/e3sm/inputdata. Then left it there as a backup.

This led to odd behavior when check_input_data run at NERSC tried to download a file and it wasn't available on any server. It created a 0 size file with the name of the file being downloaded in DIN_LOC_ROOT. Subsequent runs asking for the same file thought it was there. Wasn't until someone went and looked at the file, removed it and saw it come back that this was found (see https://github.com/E3SM-Project/E3SM/issues/5899).

I knew there was a reason why DIN_LOC_ROOT and a data server can't be the same directory but still an odd fail mode.