`x build unstable-book` is too lazy and doesn't reflect subsequent edits

[Zalathar]

(Via https://rust-lang.zulipchat.com/#narrow/stream/326414-t-infra.2Fbootstrap/topic/Is.20.60x.20doc.20unstable-book.60.20broken.3F)

If I run x doc unstable-book with a clean ./build directory, it seems to work fine.

But if I then edit or create files in ./src/doc/unstable-book, and then re-run x doc unstable-book, the command appears to succeed, but doesn't actually re-generate the book. Any edits to the book after the first build do not appear in the output.

As a workaround, I have to do rm -r build/aarch64-apple-darwin/doc/unstable-book/ to force the book to actually rebuild.

@rustbot label +T-bootstrap

[Zalathar]

cc @albertlarsan68