Instead of using the -i flag to clean out the entire set of environment
variables, we explicitly just clear out the variables that are affected by
32- vs 64-bit builds in vcvars.bat. This still allows cargo to use it's
internal autodetection to do host/target builds as it wants when cross-compiling,
but also allows other things that rely on include directories and so on to
work properly.
MozReview-Commit-ID: 99SECKv1zQa
--HG--
extra : rebase_source : 2c2a7d79201b66ac3262afcc5f060936ca2e8fa2