The appveyor.yml change bumps it for windows. The macOS worker has the
rust version bumped out-of-band and the change to .taskcluster.yml just
updates the documentation.

Differential Revision: https://phabricator.services.mozilla.com/D64945

[ghsync] From https://hg.mozilla.org/mozilla-central/rev/3ca5d53055d7f8665a29eed0b6fd6d992d834ce5