Allow supplying the MySQL password in an environment variable

I'm using xtrabackup in a production environment where we store passwords in a secure password store, and avoid storing passwords directly on disk wherever possible. For most MySQL tools -- including xtrabackup 2.4 and earlier -- we're able to pass in MySQL passwords using the `MYSQL_PWD` environment variable. This avoids the need to store them on disk (in a .my.cnf file), or specify them on the command line. However, this no longer works in xtrabackup 8.0.

Having xtrabackup 8.0 and up accept the MySQL account password via an environment variable (ideally the `MYSQL_PWD` variable used by other tools, but any variable would do).