`cargo fmt` with (pinned) nightly

[szokeasaurusrex]

cargo fmt's nightly channel offers much functionality not available with the non-nightly version, and which could make formatting across the codebase more consistent.

Let's therefore enable formatting with a nightly version of cargo fmt. To prevent random failures when the formatter is updated, we should pin a specific nightly version. As we already have a rust-toolchain.toml pinning the stable version, we can create a separate toolchain file for nightly formatting; we would copy this file to overwrite rust-toolchain.toml when running the cargo fmt jobs (multiple Rust toolchains are not natively supported, that is why we need this workaround)

[linear[bot]]

CLI-187