The GitHub action installs two Rust toolchains

[zhassan-aws]

With #26, the GitHub action now installs two Rust toolchains:

1. The stable toolchain (explicitly through the actions-rs/toolchain action)
2. Kani's integrated toolchain (through the cargo-kani setup step).

The action should instead install the toolchain needed by Kani directly. This can be done by parsing Kani's rust-toolchain.toml file.