Unpin Rust toolchain version when building old docs

We pin the Rust toolchain by way of a `.rust-toolchain.toml` file, but we mostly do so to make sure devs are all using the latest version. Rust has good backwards compatibility (often new functions etc. are added, but old code should compile with new compilers), so the latest version of the toolchain should be fine. In future, we will probably have a different pinned version of the toolchain for _every_ release and making users/GitHub runners download all of them seems silly.

It seems you can tell `rustup` to use a specific version of the toolchain [by setting the `RUSTUP_TOOLCHAIN` environment variable](https://rust-lang.github.io/rustup/overrides.html). This will override the setting in `.rust-toolchain.toml`, if any. We probably just want to set it to `stable`.

It's debateable whether we should make this a policy that is enforced both locally on CI or just on CI. Personally, I think we should probably be consistent and use `stable` for both.