MTR tests require perl-Env which is not always in the default installation

Some MTR tests use include/truncate_file.inc which, in turn, uses Env perl package. Apparently, it's not installed by default on some systems, e.g. on CentOS 7, so when we run MTR tests on our RPM packages, some of them fail due to the missing package.

It's probably not very important, if nobody complained so far. For now, I've added installation of the package to the buildbot test step, but maybe it should be pulled as a dependency for MariaDB-test.