Add [env] section to reference

[flxo]

With #9175 the [env] section is added to the configuration which allows to set environment variables from the cargo configuration. Add the [env] section with an example to the reference.

Fixes #9296

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @alexcrichton (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

Please see the contribution instructions for more information.

[flxo]

The CI failed because of network failures. Can someone retrigger?

[ehuss]

Thanks for the PR! The [env] documentation may be found here: https://doc.rust-lang.org/nightly/cargo/reference/unstable.html#configurable-env

Unstable documentation is always in the unstable chapter, and will be moved out when it is stabilized.

[flxo]

Thanks for the PR! The [env] documentation may be found here: https://doc.rust-lang.org/nightly/cargo/reference/unstable.html#configurable-env

Unstable documentation is always in the unstable chapter, and will be moved out when it is stabilized.

Ah ok. Thanks - I missed that and didn't notice when reading the contribution guides. My thought was that since the doc patch is not in the original PR with the feature that it's missing.