MSYS2 has a slightly different directory structure (binaries are
now under "/usr/bin/" instead of "/bin/"), and we're now plonking
it in `%MOZILLABUILD%\msys2` instead of `%MOZILLABUILD%\msys` so that
MSYS1 files don't interfere with MSYS2 after a pave-over install.

Speaking of pave-over installs: if both `msys2` and `msys` are available,
this patch prefers `msys2`. This is because MozillaBuild installations
with MSYS2 are going to _assume_ they're using MSYS2, and therefore
be most compatible with the versions of packages shipped with MSYS2.

Differential Revision: https://phabricator.services.mozilla.com/D133549