Auto merge of #106520 - ehuss:update-mdbook, r=Mark-Simulacrum

Auto merge of #106520 - ehuss:update-mdbook, r=Mark-Simulacrum

Update mdbook

This updates mdbook from 0.4.21 to 0.4.25. The list of changes is [here](https://github.com/rust-lang/mdBook/blob/master/CHANGELOG.md#mdbook-0425). The only user-visible changes are some changes around the theme picker, and change to the copy-to-clipboard ignoring hidden lines.

Internally there were some dependency updates and small fixes.

This also updates `clap` from 4.0.15 to 4.0.32 whose changelog is [here](https://github.com/clap-rs/clap/blob/master/CHANGELOG.md#4032---2022-12-22). This impacts tools like cargo. I don't see anything particularly noteworthy there, though there are some small user-visible changes.

Unfortunately this required adding a hack for building `rustix` with a bootstrap tool. The comment explains why. I am unable to think of some other workaround (or even a cleaner way to set the rustflag). Ideas are welcome if you can think of alternatives. I'm struggling to even think of a long-term solution, other than asking projects not to do auto-nightly feature detection.

One medium-term solution is to avoid the clap dependency for the mdbook library (which is how rustix gets pulled in). That is one of my goals for the 0.5 release of mdbook, but that probably won't happen until later this year. It would also require dropping clap from `rustbook` and using some other means to parse arguments (there's only two options, so it can probably be done manually).