Use a prebuilt copy of build-manifest

[pietroalbini]

This PR changes promote-release to use a copy of build-manifest prebuilt by CI, and stops it from cloning rust-lang/rust or invoking ./x.py at all when the prebuilt copy is detected. The PR applies the following changes:

• The channel is now represented with an enum instead of a string, which avoids having the default case in matches everywhere.
• The PROMOTE_RELEASE_OVERRIDE_BRANCH environment variable was replaced with PROMOTE_RELEASE_OVERRIDE_COMMIT, which gives more flexibility to the person starting the release. Along with that, ./run.sh added support for running a local build with a specific commit.
• A clone of rust-lang/rust is not needed anymore to detect which commit needs to be released. Instead of updating the repository and looking at the tip of the branch we're interested in, promote-release now uses git2 to connect to GitHub and list the refs available on the server, without ever receiving any git object. This is the equivalent of git ls-remote.
• The current date is now fetched with chrono instead of shelling out to the date command, since we're depending on it anyway.
• The prebuilt copy of build-manifest is now used when available, and on the channels without it the legacy invocation through ./x.py is used instead. This removes the need to clone rust-lang/rust or invoke the build system at all, speeding up releases.

This PR is best reviewed commit-by-commit.
r? @Mark-Simulacrum

[Mark-Simulacrum]

r=me. I am not too happy with constantly re-fetching the whole Rust repository but I don't know that we have better options. I definitely don't want to use github API here, for example, or use the non-rate limited alternatives.

[pietroalbini]

I am not too happy with constantly re-fetching the whole Rust repository but I don't know that we have better options.

@Mark-Simulacrum oh, to be clear, we're not fetching the whole Rust repository. git ls-remote (which I recreated with git2) just connects to GitHub and returns the list of available branches. No object data is downloaded:

pietro@january: ~/tmp/demo$ git init
Initialized empty Git repository in /home/pietro/tmp/demo/.git/
pietro@january: ~/tmp/demo (master)$ du -sh .git
104K    .git
pietro@january: ~/tmp/demo (master)$ git ls-remote https://github.com/rust-lang/rust.git | head -n 5
93deabce03dc10a80244f5da3e3819452744da2a        HEAD
fe604c5ee8fb9665e8fc4d73515a6d87f05997ab        refs/heads/LeSeulArtichaut-patch-1
bbea1a1f7b4a32a15259becce7800bf7e0cad423        refs/heads/auto
212e76c31d461462544b5d516d3bdc99fc8f016e        refs/heads/beta
93deabce03dc10a80244f5da3e3819452744da2a        refs/heads/master
pietro@january: ~/tmp/demo (master)$ du -sh .git
104K    .git
pietro@january: ~/tmp/demo (master)$ 

[Mark-Simulacrum]

Hm, I recall seeing a git rev-parse which I wouldn't expect to work in that case.