Fixes #3160 - Backup script restoration order causes issues if version difference causes ruby version differences

This commit ensures that the restore script will restore the database **before** restoring the file system of Zammad.

This solves ruby errors for environments that don't have old ruby versions installed (because they shouldn't be required during restorations).

This commit provides a compromise and behaves the old way for docker and source code installations. This is because it may be a source code installation is restored right away and was not installed before.

Hints for this are prepared for the documentation over at: https://github.com/zammad/zammad-documentation/pull/140

contrib/backup/functions

@@ -153,10 +153,10 @@ function stop_zammad () {
 }
 
 function restore_zammad () {
-  echo "# Restoring Files"
-  tar -C / --overwrite -xzf ${BACKUP_DIR}/${RESTORE_FILE_DATE}_zammad_files.tar.gz
-  echo "# Ensuring correct file rights ..."
-  chown -R zammad:zammad ${ZAMMAD_DIR}
+  if ! command -v zammad > /dev/null; then
+    # See #3160 for the reasons of this :>
+    restore_files
+  fi
 
   if [ "${DB_ADAPTER}" == "postgresql" ]; then
     echo "# ... Dropping current database ${DB_NAME}"
@@ -185,6 +185,18 @@ function restore_zammad () {
     echo "# Restoring MySQL DB"
     zcat ${BACKUP_DIR}/${RESTORE_DB_DATE}_zammad_db.${DB_FILE_EXT}.gz | mysql -u${DB_USER} -p${DB_PASS} ${DB_NAME}
   fi
+
+  if command -v zammad > /dev/null; then
+    # See #3160 for the reasons of this :>
+    restore_files
+  fi
+}
+
+function restore_files () {
+  echo "# Restoring Files"
+  tar -C / --overwrite -xzf ${BACKUP_DIR}/${RESTORE_FILE_DATE}_zammad_files.tar.gz
+  echo "# Ensuring correct file rights ..."
+  chown -R zammad:zammad ${ZAMMAD_DIR}
 }
 
 function start_backup_message () {