rustup-init with rust-toolchain

[XAMPPRocky]

Describe the problem you are trying to solve
With the introduction of the TOML version of rust-toolchain, there isn't an easy way to install the exact version and components without also first installing the default toolchain. Where as with the old one line version you could write rustup-init --default-toolchain $(cat rust-toolchain).

Describe the solution you'd like
rustup-init --from-file ./rust-toolchain or something similar.

[rbtcollins]

Dup of #2686 with the mild tweak of permitting it during rustup-init.