MDEV-25207 mysql_install_db doesn't create sys schema (fix)

Fix for out-of-tree builds.

mysql_sys_schema.sql is a CMAKE_CURRENT_BINARY_DIR file so
use the buildpkgdatadir.

scripts/mysql_install_db.sh

@@ -388,7 +388,7 @@ create_system_tables2="$srcpkgdatadir/mysql_performance_tables.sql"
 fill_system_tables="$srcpkgdatadir/mysql_system_tables_data.sql"
 maria_add_gis_sp="$buildpkgdatadir/maria_add_gis_sp_bootstrap.sql"
 mysql_test_db="$srcpkgdatadir/mysql_test_db.sql"
-mysql_sys_schema="$srcpkgdatadir/mysql_sys_schema.sql"
+mysql_sys_schema="$buildpkgdatadir/mysql_sys_schema.sql"
 
 for f in "$fill_help_tables" "$create_system_tables" "$create_system_tables2" "$fill_system_tables" "$maria_add_gis_sp" "$mysql_test_db" "$mysql_sys_schema"
 do