MT#57270 Don't wipe sql dump header

For sql dump we need to setup some variable like FOREIGN_KEY_CHECKS=0,
or NAMES utf8 as well as 'CREATE DATABASE' statement. And we do but then
it's wiped due to incorrect redirection '>' instead of '>>'. So fix it.

Change-Id: Id37153d5e4ac038f788c34443e1bb5715d5e3985

build_utils/ngcp-build-schema-files

@@ -101,7 +101,7 @@ for schema in "${schemes[@]}"; do
   mysqldump "${schema}" --compact --skip-extended-insert \
     -S "${mariadb_dir}/mysqld.sock" \
     | sed -r "s/'[[:digit:]]{4}-[[:digit:]]{2}-[[:digit:]]{2} [[:digit:]]{2}:[[:digit:]]{2}:[[:digit:]]{2}'/NOW()/g" \
-    > "${schema}.sql"
+    >> "${schema}.sql"
   echo "Done"
 done