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.
   • 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