ERROR mpassi buildnml: missing specified restart file for branch or hybrid run

When using run_e3sm script, and a branch run is started (ie, creating a newcase that restarts from files in another case), the new case will have soft-links pointing to the restart files in the previous case. This is needed for the first run. However, once the new case continues, the files in the previous case should not be needed, yet if they are removed, we get the following error:

```
Running /lus/theta-fs0/projects/ClimateEnergy_4/ndkeen/wacmy/E3SM_code/20190910-maint-1.0/components/mpas-seaice/cime_config/buildnml
ERROR mpassi buildnml: missing specified restart file for branch or hybrid run: /projects/ClimateEnergy_4/ndkeen/E3SM_simulations/theta.20190910.branch_noCNT.n825def.unc06.A_WCYCL1950S_CMIP6_HR.ne120_oR
RS18v3_ICG/run/mpascice.rst.0056-01-01_00000.nc at /lus/theta-fs0/projects/ClimateEnergy_4/ndkeen/wacmy/E3SM_code/20190910-maint-1.0/components/mpas-seaice/cime_config/buildnml line 170.

env_run.xml:
env_run.xml:

if (( $RUN_TYPE eq 'hybrid') || ( $RUN_TYPE eq 'branch' )) {
$input_file = "$RUNDIR/mpascice.rst.${RUN_REFDATE}_${RUN_REFTOD}.nc";
if (! -e "$input_file") {
die " ERROR mpassi buildnml: missing specified restart file for branch or hybrid run: $input_file ";
}
}
```

I wanted to make a note of this as it might come up again, but wasn't sure where to report it.

A suggestion was made by @jonbob to make the buildnml scripts check for CONTINUE_RUN and then not look for those files if the run is restarting on its own -- where the REFCASE files are no longer required.