Auto merge of #77762 - pietroalbini:dist-build-manifest, r=Mark-Simulacrum

Dist build manifest

This PR makes two changes that should remove a significant chunk of the time spent in our release process: cloning the `rust-lang/rust` monorepo, all its submodules, and building `bootstrap` to then invoke `build-manifest`:

* `build-manifest` doesn't rely on a clone of the monorepo being present anymore. The only remaining bit of information it fetched from it (the Rust version) is instead bundled in the binary.
* A new "component" is added, `build-manifest`. That component includes a prebuilt version of the tool, and it's *not* included in the Rustup manifest. This will allow `promote-release` to directly invoke the tool without interacting with our build system.
* The Linux x86_64 CI is changed to also build the component mentioned above. It's the only CI builder tasked to do so, and to cleanly support this a new `--include-default-paths` flag was added to `./x.py`.
* The `BUILD_MANIFEST_NUM_THREADS` environment variable is added to configure the number of threads at runtime.

This PR is best reviewed commit-by-commit.
r? `@Mark-Simulacrum`

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