Ensure rustup is up-to-date to avoid users having a version that is too
old to support the TOML format.
If rustup was just installed, tell the user to source the new env vars
to use the correct version of the toolchain.
Signed-off-by: Tim Crawford <tcrawford@system76.com>
rustup 1.23.0 (2020-11-27) introduced support for TOML syntax for the
toolchain file. Use this and specify required compoenents.
To ensure you are using a new enough rustup, run:
rustup self update