precheck test of restart existence doesn't handle multi-instance

[MOM_interface](https://github.com/ESCOMP/MOM_interface/tree/main)/[cime_config](https://github.com/ESCOMP/MOM_interface/tree/main/cime_config)/buildnml lines 403-405 check for the existence of the MOM6 restart file
for a single instance; ${CASE}.mom6.r.${DATE}.nc, even when it's a multi-instance case.
It fails in that case because the necessary restart files have the instance numbers in them:
${CASE}.mom6.r.${DATE}.\_${INST}.nc (old, default form) or
${CASE}.mom6\_${INST}.r.${DATE}.nc (new form).

I suggest that the check for restart file(s) be put into the if-blocks earlier in the function.
I don't know the right combinations of testcase, run\_type, continue\_run, inst\_suffixes, etc.
to use in the conditionals, so I don't have a proposed improved function.