Prepare CI workflow for nightly releases

[Kobzol]

This new CI workflow builds a rustc-perf site binary each day (night) and publishes it into a draft GH release. The release is overwritten each day, so we won't be spammed with hundreds of releases.

[Kobzol]

Here is the built release: https://github.com/rust-lang/rustc-perf/releases/tag/nightly.

[Kobzol]

Rebased over master. Once this is merged, we should be free to start using npm.

[Kobzol]

I'd like to merge this before #1565 so that we can provide a nightly release for users that do not want to download npm to build the website.

Ping @Mark-Simulacrum

[Mark-Simulacrum]

This seems okay for now. Let's see how it works out in practice, we may want to put the site binary into our perf S3 bucket as an alternative to GHA if there's any problems with the overwriting here.

[Kobzol]

I use a similar approach in a different repo and it works fine I think. The only issue is that the overwritten nightly release still keeps the "publication date" of the first release (so it looks like the release was published a long time ago), but that's just a cosmetic GitHub UI thing, the textual description of the release clearly states the date of the nightly.