Support +toolchain rustup override in bash completions.
Support +toolchain rustup override in bash completions.

Fixes #5107

tsurai/cargo,vojtechkral/cargo,nipunn1313/cargo,tshepang/cargo,tsurai/cargo,tshepang/cargo,rustyhorde/cargo,rustyhorde/cargo,rustyhorde/cargo,rustyhorde/cargo,tsurai/cargo,vojtechkral/cargo,rust-lang/cargo,nipunn1313/cargo,nipunn1313/cargo,rust-lang/cargo,tshepang/cargo,vojtechkral/cargo,tshepang/cargo,rust-lang/cargo,tsurai/cargo,nipunn1313/cargo,rust-lang/cargo,tsurai/cargo,eminence/cargo,vojtechkral/cargo,eminence/cargo,eminence/cargo,eminence/cargo