Adds:
- arm64-apple-darwin
- x86_64-linux-gnu
- x86_64-w64-mingw32
... Guix builds to CI as a standalone workflow.
Testing exposed that manifest_build.scm depends are invalidated by a subsequent manifest_gui.scm build, so the first commit here splits these inside of BASE_CACHE. This means they can both be cached and re-used.
The second commit exports the time machine commit which makes it easier to re-use in other scripts e.g. this CI workflow, but also places like an automated Guix substitute build server.
The final commit adds the new workflow and guix install shell snippet.