Historically, a mozinfo for js standalone build has not been necessary,
but with the move towards a) having things work with mach and b)
buildconfig using the MozbuildObject.from_environment in next patch,
mozinfo becomes necessary (it's required for
MozbuildObject.from_environment to find the directory is an objdir).
Interestingly, hazard builds do both a js standalone build and a desktop
Firefox build at the same time, both of which are done with MOZCONFIG
set in the environment... with the Firefox mozconfig. The result of now
writing a mozinfo for js standalone builds is that in that case, they
end up with a reference to the mozconfig, which the build system then
reads, and finds a MOZ_OBJDIR, which then doesn't match the js
standalone build objdir. So for those builds, reset MOZCONFIG.
