MDEV-14572: Assertion `! is_set()' failed in Diagnostics_area::set_eof_status upon EXPLAIN UPDATE in PS

Restore EXPAIN flag in SELECT_LEX before execution multi-update by flag in LEX
(the same but in other way made before INSERT/DELETE/SELECT)

Without it, mysql_update() didn't know that there will be EXPLAIN result set and was sending OK at the end of the update, which conflicted with the EOF sent later by EXPLAIN.