Add the ability for Kani to use a locally installed toolchain at runtime

[jaisnan]

In order to use GPG verified toolchain components, Kani needs to be able to use locally installed toolchains. These toolchains are typically fetched from the rust-lang server and then, verified with the associated .asc file.

Kani's setup currently depends on rustup to install the toolchain, which doesn't allow us to use verified components. This also means having rustup itself as a dependency.

Being able to do this, will also enable us to remove rustup as a runtime dependency.