How to tell "use older parts in toolchain"?
Yeah, probably a Rust newbie question, still a sincere question.
Was a older version installed? How to tell `rustup` to keep old
versions? Was done with a cargo.toml entry? Or with file
`rust-toolchain`?  Please tell   (Please spoon feed me ;-)

Using `rust-toolchain.toml` should work.
Another option would be to override the default version for the given
directory with
`rustup override set nightly-2021-02-20`.

You could also set the global default `rustc` to a specific nightly release:
`rustup default nightly-2021-02-20`.

More info here: https://rust-lang.github.io/rustup/overrides.html