Setting `-ex` in the shebang makes it a little hard to see since folks
are used to glancing up and looking for `set -ex`. Both styles achieve
the same result, so this does not change the logic of the script.

shellcheck emits:

 SC2027: The surrounding quotes actually unquote this. Remove or escape them.

Remove inner quotes from shell variable so the feature is correctly printed.

Cargo already has an env var for controlling the toolchain, we should
use it instead of our current custom `TOOLCHAIn`.

While we are at it fix the missing `$` from variables (found by
`shellcheck`).

Use RUSTUP_TOOLCHAIN to allow control of the toolchain.