Auto merge of #125856 - onur-ozkan:bootstrap-submodule-hotfix, r=onur-ozkan

include missing submodule on bootstrap

As of rust-lang/rust#125408 PR, rustbook now relies on dependencies from the "src/doc/book" submodule.

However, bootstrap does not automatically sync this submodule before reading metadata informations. And if the submodule is not present, reading metadata will fail because rustbook's dependencies will be missing.

This change makes "src/doc/book" to be fetched/synced automatically before trying to read metadata.

cc `@Zalathar`

Author: bors