When the user installs Rust tools using a method other than rustup, e.g. using
an operating system's package manager, cargo generally has no support for
toolchain specification. In such case, the user can then `nil', or `""', so that
the respective functions in `rustic-babel' will remove the toolchain from
params, i.e. only toolchain has a valid value if Cargo has toolchain support.

See also: brotzeit/rustic#279 (comment)

Fixes #498 introduced in 80d05c4

Co-authored-by: Sibi Prabakaran <[email protected]>

We are setting the default to `nil`, which means use the default provided by
cargo and don't specify it explicitly. If overridden with a string value, that
will be used instead and passed as +<toolchain>.