Automatically create prelease releases on master builds

[reese]

This adds a new GH action that runs on non-tagged master builds. It basically makes sure tests still pass, then it bumps the version tag to a new pre-patch version and runs all the release scripts to create a prerelease build. (Note that this doesn't generate an arm Darwin build -- I tried to add the runner for it and got some complaint about billing so I assume that's only a paid thing -- but it builds the x86 and arm Linux builds, which is mostly what I care about.)

The reason to do this is that it makes it far easier to iteratively pull and use new rubymt versions as we make changes. In the past, manual version bumps cause a lot of toil, so we didn't bump our rubyfmt very often. In an ideal world, we would bump even on patch versions to stay as close to HEAD as possible. Since they're prereleases, this won't cause updates to homebrew or anything -- it's only available by pulling the GH release, and we can still cut manual tag releases for homebrew releases as needed.