Make RERUN receive value from out of the script

This is mostly for GEFS forecast_d16_35 job

 On branch feature/gefs_v13_822_add_atm_fcst
	modified:   ush/forecast_det.sh

Refs: NOAA-EMC#822

ush/forecast_det.sh

@@ -39,7 +39,7 @@ FV3_GFS_det(){
 
   #-------------------------------------------------------
   # determine if restart IC exists to continue from a previous forecast
-  RERUN="NO"
+  RERUN=${RERUN:-"NO"}
   filecount=$(find "${RSTDIR_ATM:-/dev/null}" -type f | wc -l)
   if [[ ( ${CDUMP} = "gfs" || ( ${RUN} = "gefs" && ${CDATE_RST} = "" )) && ${rst_invt1} -gt 0 && ${FHMAX} -gt ${rst_invt1} && ${filecount} -gt 10 ]]; then
     reverse=$(echo "${restart_interval[@]} " | tac -s ' ')