[ci] Fix broken "build docs" in nightly_docs.yml

[notmandatory]

There is currently a regression in +nightly rustdocs that is breaking our CI docs build.

I believe this is caused by rust-lang/rust#93476

I think we have two options:

1. For the docs CI jobs remove the --cfg docsrs flag and build docs with +stable
2. Wait for rustdocs in +nightly to be fixed

[notmandatory]

I prefer option 1, even though we won't see which feature is required to enable feature gated code, we can keep documenting it with the docsrs attribute and when the feature(doc_cfg) is available in +stable turn it back on.

If we take option 2 the issue may get fixed in the next few days but we can still run into some other regression in a future +nightly and will have our CI randomly broken again.

[afilini]

Maybe a third option is to pin a specific nightly version that we know is working fine?

It's not the end of the world to lose the doc_cfg feature, but I would like to replicate the environment in docs.rs as much as we can, so that we can see if anything is wrong before publishing a release

[notmandatory]

Good idea! I forgot we can use specific nightly builds. I'll put together a PR to do that.