Sanitizer builds require unstable features from the rust compiler, and
the common way to achieve this is to use a nightly rust compiler,
which we had been doing for a while.
Later on, we also needed to use unstable features that actually require
some patches to both rustc and cargo to work properly for our needs, so
we switched to taking the source of a nightly rust compiler, patch and
build it, and use that.
But finding an appropriate nightly rust compiler is not always easy. We
want something close enough from the stable rust compiler we're using
otherwise, which we've been using the first nightly that has the
following version number for. But it also can happen that those versions
have bugs that have been fixed later and then uplifted during the beta
period (and thus fixed in the stable rust compiler).
On the other hand, taking the source of the stable rust compiler and
compiling it with a `channel` set to `dev` (actually the default) or
`nightly` yields a compiler that is very close to the actual stable rust
compiler, but acts like a nightly compiler and allows to enable unstable
features.
So instead of trying to pick a good version from the nightly channel,
just use the same version as the stable rust version we use.
Differential Revision: https://phabricator.services.mozilla.com/D151341
a753b3db0b