`rustup ...` no longer installs override toolchain verison

[nhtyy]

Previously, I believe, that if you were in a working directory with a valid rust-toolchain{.toml} then running any rustup command would install the overriden toolchain. However i've been experincing CI failures to due to this seemingly being no longer the case.

[jieyouxu]

Not for this issue tracker, rust-lang/rustup#4211.