Modified buildnml script

During the DART+CAM6 Reanalysis, Kevin Raeder and Jeff Steward identified and fixed a bottleneck in one of the MCT build scripts:

/glade/work/raeder/Models/cesm2_1_relsd_m5.6/cime/src/drivers/mct/cime_config/buildnml

The script runs slowly for a multi-instance CESM case because it attempts to write multiple files simultaneously into directories that are locked.

This bottleneck is inevitable during the first submission of case.run, but it can be avoided on subsequent submissions.

cd /glade/work/johnsonb/cesm2_1_3/cime/src/drivers/mct/cime_config
git mv buildnml buildnml.old
cp /glade/work/raeder/Models/cesm2_1_relsd_m5.6/cime/src/drivers/mct/cime_config/buildnml ./