add Github Action for Cosmopolitan Libc build

[ahgamut]

This PR adds a Github Actions workflow to build cxx from source with Cosmopolitan Libc with every release.
The resulting binary should work on x86_64 (Linux/BSDs/Windows/MacOS), and aarch64 (Linux/MacOS) as per the Cosmopolitan v3.3.3 README.

An example cxx binary is available here: https://github.com/ahgamut/cxx/releases/tag/untagged-41b8dd7f7a169bcd65bf download and chmod +x (or rename to cxx.exe on Windows) to run the executable from the command-line.

[xyproto]

Thanks for the PR.

How is the dependency on scons solved, though?

[ahgamut]

Hmm, I guess for now scons will have to be available on the user's $PATH, although we could just build scons with Cosmopolitan Libc as well.