Fixed use of incorect var for OS detection

SubModules/postfix/preconf.sh

@@ -7,7 +7,7 @@ if [ -z "${domain}" ]; then
     fi
 fi
 
-if [ "$shortdist" = "ubu1804" ] || [ "$osrel" = "ubu2004" ] || [ "$osrel" = "deb10" ] ; then
+if [ "$shortdist" = "ubu1804" ] || [ "$shortdist" = "ubu2004" ] || [ "$shortdist" = "deb10" ] ; then
     debconf-set-selections <<< "postfix postfix/mailname string $domain"
     debconf-set-selections <<< "postfix postfix/main_mailer_type string 'Internet Site'"
 fi