Include `llvm-tools-preview` in `rust-toolchain.toml`

[hawkw]

Building the bootloader test and example kernels requires the llvm-tools-preview Rust toolchain component to be installed. Rustup can automatically install additional toolchain components if the rust-toolchain file includes a components section.

This commit adds a configuration in the rust-toolchain file to tell Rustup to install llvm-tools-preview automatically. This requires switching the toolchain file from the legacy format to the TOML format. Therefore, I've also renamed the file from rust-toolchain to rust-toolchain.toml, so that it gets highlighted as a TOML file by most editors by default.

[Freax13]

This will run into https://github.com/actions-rs/toolchain/issues/126

[phil-opp]

This will run into actions-rs/toolchain#126

With all components specified in the rust-toolchain.toml we should not need the toolchain action anymore. The GitHub action runners include rustup by default, which will automatically install the specified toolchain and components. The next branch already includes this change, maybe it's possible to cherry-pick.

[hawkw]

@phil-opp I can cherry-pick the changes to the CI configuration, one sec.

[phil-opp]

This was implemented as part of the v0.11 rewrite. Thanks for proposing this!