always use stable Rust to build josh

[RalfJung]

I am not a huge fan of having josh installed automatically -- that's a global change to my system that IMO a tool shouldn't do without first asking me, and it makes it impossible to use the tool with a local, experimental version of josh (this cargo install will always downgrade it back to the released version). However, I am even more not a fan of having that build done with whatever random toolchain happens to be active when I invoke josh-sync. At the very least, let's make sure we use a stable version of Rust, not some nightly snapshot.

[Kobzol]

FWIW, one of the original motivations for this tool was to ensure that everyone will be using the same version of Josh, and more specifically a version that is up-to-date, as we all remember what happened when an out-of-date version was used :D Having a CLI opt-out to not install josh would be fine.