Guide.md: Change download instructions and remove bad info

Piping `curl` directly to `| sudo sh` has very weird failure modes:  If the connection is interrupted, you can end up executing part of a script.

The small rant about "Trusting Rust" to not hack your computer is completely misguided as well.  You're going to run binaries one way or another, so you're still trusting Rust developers.

The fact that we're delivering unsigned binaries over http is still cringeworthy though.