The mariabackup interface currently supports passing a password
through an explicit command line variable, but does not support
passing a password through the MYSQL_PWD environment variable.
At the same time, the Galera SST script for mariabackup uses
the environment variable to pass the password, which leads
(in some cases) to an unsuccessful launch of mariabackup and
to the inability to start the cluster. This patch fixes this
issue. It does not need a separate test, as the problem is
visible in general testing on buildbot.