Bug 1560527 was not supposed to change verbosity for mach build, but it
turns out it did, because the ifeq it copied from one place to another
was wrong in the first place.

While here, replace a ifeq that did work with the now equivalent
BUILD_VERBOSE_LOG.

Differential Revision: https://phabricator.services.mozilla.com/D35966

--HG--
extra : moz-landing-system : lando