Tracking issue: Rustbuild submodule management can break nightlies about rust-central-station

[alexcrichton]

Thanks for making an issue! So the surface problem here is likely mostly taken care of and is unlikely to resurface (submodule management). The core problem though is that producing a nightly requires running processes that are not tested as part of our CI. This means that things can break!

Overall to really squash this bug we need two separate pieces:

• As part of rust-lang/rust CI we need to produce the manifests that rust-central-station produces. I don't know how to do this with today's setup.
• We also need to CI rust-central-station itself as part of the rust-lang/rust build to ensure that everything works out in terms of deployment and whanot.
• Finally, we need more integration testing on rust-central-station itself to run it through its paces when making changes.

Unfortunately I don't really know how to do any of those. I was thinking we could add a CI builder which just publishes an older set of artifacts as a release to a dummy location, but that's not really solving the above problems because it's not (a) testing the master branch of rust-central-station or (b) testing the artifacts that are about to be produced by the build.

[pietroalbini]

This is fixed by the new promote-release, as it doesn't invoke rustbuild nor clone rust-lang/rust anymore. The new promote-release also features testing in CI.