Fixed issue with using pg_upgrade on ubuntu-16 hub installer upgrade

Issue is that ubuntu-16 has problems with pg_upgrade due to backup/bin/postgres
"cannot execute" during it's checks.
This is fine since we have other ways to migrate that work.

Ticket: ENT-10710
Changelog: title

packaging/common/cfengine-hub/postinstall.sh

@@ -699,11 +699,15 @@ do_migration() {
       exit 0 # exits only from (...)
     fi
     cf_console echo "Migration using pg_upgrade failed."
-    # here pg_upgrade probably said something like
-    # Consult the last few lines of "/var/cfengine/state/pg/data/pg_upgrade_output.d/20230913T150025.959/log/pg_upgrade_server.log" for the probable cause of the failure.
-    cf_console echo "Showing last lines of any related log files:"
-    _daysearch=$(date +%Y%m%d)
-    find "$PREFIX"/state/pg/data/pg_upgrade_output.d -name *.log | grep "$_daysearch" | cf_console xargs tail
+    # skip ubuntu-16 since there is an expected failure there
+    # check for "/var/cfengine/state/pg/backup/bin/postgres" failed: cannot execute
+    if [ "$BUILT_ON_OS" != "ubuntu" ] && [ "$BUILT_ON_OS_VERSION" != "16" ]; then
+      # here pg_upgrade probably said something like
+      # Consult the last few lines of "/var/cfengine/state/pg/data/pg_upgrade_output.d/20230913T150025.959/log/pg_upgrade_server.log" for the probable cause of the failure.
+      cf_console echo "Showing last lines of any related log files:"
+      _daysearch=$(date +%Y%m%d)
+      find "$PREFIX"/state/pg/data/pg_upgrade_output.d -name *.log | grep "$_daysearch" | cf_console xargs tail
+    fi
     cf_console echo
     check_disk_space # will abort if low on disk space
     init_postgres_dir "$new_pgconfig_file" "$pgconfig_type"