pkg/backup: Use --flush-privileges when backing up MariaDB
pkg/backup: Use --flush-privileges when backing up MariaDB

This emits a `FLUSH PRIVILEGES` command at the end of the dump to ensure
information restored from the `mysql` database is made active.

Closes #3394

Signed-off-by: Joseph Glanville <aed65cdb66c1129f81e57d35b788d35820e08ea4@jpg.id.au>

philiplb/flynn,flynn/flynn,flynn/flynn,flynn/flynn,flynn/flynn,flynn/flynn,lmars/flynn,lmars/flynn,philiplb/flynn,lmars/flynn,philiplb/flynn,philiplb/flynn,lmars/flynn,lmars/flynn,philiplb/flynn