Bug 1802405 - Prepend mozillabuild msys2 to path only for the build. r=glandium

This reverts bug 1801826 and instead prepends the msys2 path only for
the build, which is a bit more sensible.

This allows us to remove the existing warning from old_configure, since
that can't happen now.

Differential Revision: https://phabricator.services.mozilla.com/D163073