[DO NOT MERGE] Build with rustls instead of OpenSSL

[charlespierce]

Experimenting how using Rustls affects final binary size

[charlespierce]

Looks like ring doesn't like to compile on CentOS 6 since it's so old, but that's the level we've been supporting, so using RusTLS would mean changing our supported environments.

[chriskrycho]

@charlespierce @dherman @thoov @rwjblue thoughts on leaving behind CentOS 6 as one of the things we do with 1.0? Even if that means we cut a 0.10.0 with support for it first, and then 1.0.0 which drops it (and does e.g. this if it's useful).

[charlespierce]

@chriskrycho It's not specifically CentOS 6, we were using that to get the CentOS flavor of OpenSSL, however using an older docker image allows us to create a build linked against an older libc version than the oldest GH Actions runner supports (Ubuntu 16.04). That makes Volta work on a wider range of Linux distros out of the box.

I'm hesitant to drop that support, since there's such a variety of distros and support timeframes. For reference, I believe rustup is actually built against CentOS 5, to support an even wider range of possible distros. If, in the future, we decide we want to use RusTLS, we can probably do it in a backwards-compatible way by creating a feature flag and still building the old OpenSSL versions on an older image. It will require making the installer script a little smarter, but that shouldn't be too hard.

[chriskrycho]

Oh, I see. Thank you for the clarification!