…=nical

This change is a temporary fix to not set RUSTFLAGS='--deny warnings'
in order to unblock https://bugzil.la/1575648 (which is time-critical).
The longer term solution for this is https://bugzil.la/1564873.

Differential Revision: https://phabricator.services.mozilla.com/D52360

[wrupdater] From https://hg.mozilla.org/mozilla-central/rev/c61c05f3707767aaf77f7cffa741c86ef44ace2d