FST: measure the AO2D size portably

stat -c%s is GNU syntax; BSD stat, as on macOS, rejects it, so the ASYNC
check read an empty size and failed with "AO2D file (AO2D.root) exists
but is empty" on a 470 kB AO2D. wc -c works on both.

Author: ktf

prodtests/full_system_test.sh

@@ -337,7 +337,7 @@ for STAGE in $STAGES; do
    # Check AOD production for ASYNC stage
   if [[ "$STAGE" = "ASYNC" ]]; then
     if [[ -f "AO2D.root" ]]; then
-      aod_size=`stat -c%s AO2D.root`
+      aod_size=$(wc -c < AO2D.root)
       if [[ $aod_size -gt 0 ]]; then
         echo "AO2D file produced: AO2D.root (size: ${aod_size} bytes)"
         echo "aod_size_${STAGE},${TAG} value=${aod_size}" >> ${METRICFILE}