Promotion should use a consistent date and time

[aidanhs]

The sign-and-hash invocation on x.py calls date, but so does RCS on startup. If these straddle a new day, bad things happen.

One big hammer is to use faketime to fix the time at the top level and make every date command return the same value.

[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.