Meta: Ship builds from the `master` branch as well

[rami3l]

Do you mean that users will be able to install builds from the master branch? We haven't thought about it yet, but I see no reason why we can't make the S3 bucket publicly available so that users can do something like RUSTUP_UPDATE_ROOT=https://builds.rustup.rs/${commit} rustup self update (or another domain)... 🤔

@jdno Yup! I had to provide CI uploads to users in order to know if some recent fix has worked for them or not (e.g. #3708 (comment)). This would be a nice addition if possible, and it seems that there shouldn't be introducing too much overhead for you?
#3931 (comment)

#3931 already uploads master-based builds to an S3 bucket, so it looks a reasonable step forward?

Although I'm not sure if one link per commit is worth it. I was originally thinking about a new release channel apart from the current "beta" and "stable".

I'm thinking about this because we of course need a whole bunch of testing for v1.28.0 (I'm dogfooding it via brew, but that's far from enough), and the longer the test-driving builds are available, the more I feel confident about the maturity of the new version :)

cc @djc @rbtcollins