Tracking: Limit the range of valid toolchain names, take caution in `rust-toolchain.toml`

[rami3l]

As discussed in #4053 (comment), there are basically two issues to address:

1. Improve current error messages and treat toolchain names coming from rust-toolchain.toml as untrusted.
2. Come up with more restrictive rules in respect to legal toolchain names.
   a. If the rules allow the prevention of phishing, lift the restriction on displaying toolchain names from rust-toolchain.toml if they are legal.

Tasks

• Toolchain Channel Manipulation within Rustup #4053