Update ci-automation/az_sig_publish.sh

Co-authored-by: James Le Cuirot <jlecuirot@microsoft.com>

Author: sayanchowdhury

ci-automation/az_sig_publish.sh

@@ -33,7 +33,7 @@ function _az_sig_publish_impl() {
 
   local date=""
   if [[ "$vernum" == *nightly* ]]; then
-    date=$(echo "${vernum}" | cut -d '-' -f4)
+    date=$(cut -d '-' -f4 <<< "${vernum}")
     FLATCAR_GALLERY_VERSION="${version%.*}.${date:2}"
   else
     date=$(date +'%y%m%d')