Auto merge of #53715 - pietroalbini:missing-components-manifest, r=alexcrichton

Auto merge of #53715 - pietroalbini:missing-components-manifest, r=alexcrichton

Include missing tools in the manifest and mark them as unavailable

This PR changes the `build-manifest` tool to always include the missing components in the manifest, marking them as `available = false`. This blocks rustup from updating to a different nightly if the component is installed.

The code builds and _should_ be correct, but I don't know a way to test the changes locally.

r? @alexcrichton
cc @kennytm https://github.com/rust-lang-nursery/rustup.rs/issues/1486