[ci] Cache build folder to save CI time

[uilianries]

Thanks to the PR #54, we can build a minimal Yocto image in the CI, so we have a complete test.

Building a yocto image for the first time take hours, not only because it builds everything from source, but also need to download all external dependencies.

The first build took >5h and it will be repeated for each new build.

It would be nice if we could cache the build folder, so we could decrease the build time to few minutes only.

The build folder should result in giga bytes, and we will need to compact it to another format (tar.xz), then store it (Arfactory?)

A blog post about sharing Yocto sstate and download folder:

https://bootlin.com/blog/yocto-sharing-the-sstate-cache-and-download-directories/