Add build-9x workflow

[2moe]

Let github actions build the rust9x toolchain automatically, then pack+compress it to tar.zst and upload it to github releases.

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The reason for using tar.zst, instead of 7z, is that 7z will give a warning when packing:

WARNING: The name of the file cannot be resolved by the system.

[seritools]

Hey, thanks for setting it up!

I'll keep it open for now and have a closer look later, when more changes to rust9x itself are necessary. I'm lucky enough to have a fast PC, so creating a stage 2 build takes just a couple of minutes locally.

I'd like to see these changes:

• Trigger the build on creating a release, instead of having the build create the release
• Build stage 2 for the dist artifacts. What I do locally is to use python x.py install, see the [install] area in config.rust9x.toml. The command will build and move everything nicely into the designated dir, so it's easy to compress
• Get 7z back running, as it's probably got much wider support than tar.zstd

[2moe]

1. Now it's triggered by "tag". Once you create and push a new tag, it is automatically released. In my opinion, triggered by tag would be better than a manual release to trigger it.

2. I changed it to python x.py install ${{env.BUILD_ARGS}}

3. About tar.zstd: The Win11(23H2) Explorer.exe and C:/Windows/system32/tar.exe can now decompress the tar.zst format. (Win11 23H2 with bsdtar pre-installed.) Nevertheless, I changed it back to 7z.

4. Consider that actions(Standard GitHub-hosted runners) are a bit slow to build. If you don't plan to use self-hosted runner, then feel free to close this PR.

[seritools]

Consider that actions(Standard GitHub-hosted runners) are a bit slow to build.

yeah that was my other concern ^^ I don't expect too many updates/constant releases, so doing it manually while testing things locally should be fine at the moment. Should it change (e.g. lots of pull requests that need CI testing) additional process can be added.