Test generated manifests against rustup

[Mark-Simulacrum]

Refiling rust-lang/rust#41767 here.

Several times, PRs have changed the format of the manifests in such a way that they can no longer be installed by rustup. This results in the corresponding nightlies being uninstallable, and doesn't give much time to fix it before the next nightly: rust-lang/rustup#1092 (comment)

We should add tests to catch these problems before they occur.

[pietroalbini]

Duplicate of rust-lang/promote-release#15.