Update tooltool manifests to reference rustc 1.16.0 stable repacks
which include cargo 1.19.0 nightly builds from 2017-04-19 which
support the RUSTC_WRAPPER environment override.
We're shipping with an unstable cargo while this feature makes its
way to release because it is necessary for deploying sccache support
for rust language code in our build and test automation.
MozReview-Commit-ID: Iow2894OPq7
c45f4c8f2a