Fix DBN alert path in enkf fcst

The filepath in the DBN alert in enkf fcst had not been updated to
use `$RUN` instead of `$CDUMP`.

Refs: NOAA-EMC#761

scripts/exgdas_enkf_fcst.sh

@@ -204,7 +204,7 @@ for imem in $(seq $ENSBEG $ENSEND); do
      while [ $fhr -le $FHMAX ]; do
        FH3=$(printf %03i $fhr)
        if [ $(expr $fhr % 3) -eq 0 ]; then
-         "${DBNROOT}/bin/dbn_alert" MODEL GFS_ENKF "${job}" "${COM_ATMOS_HISTORY}/${CDUMP}.t${cyc}z.sfcf${FH3}.nc"
+         "${DBNROOT}/bin/dbn_alert" MODEL GFS_ENKF "${job}" "${COM_ATMOS_HISTORY}/${RUN}.t${cyc}z.sfcf${FH3}.nc"
        fi
        fhr=$((fhr+FHOUT))
      done