Promotion should use a consistent date and time about rust-central-station

[aidanhs]

cc @Mark-Simulacrum

[Mark-Simulacrum]

I think perhaps the better way to fix this is to pull https://github.com/rust-lang/rust/tree/master/src/tools/build-manifest into this repo and then depend on it directly.

This will simplify x.py I think since we can then remove some configuration and a few other things -- HashSign is a really odd step anyway as-is today.

It's not completely trivial since there's some version ready x.py knows of but should be feasible and a good step to take.

[pietroalbini]

This is fixed by the new promote-release, since it passes the date it's using directly to build-manifest.