Basic CI

[attilaolah]

Set up a very basic CI that e.g. does a bazel query //... to make sure that at least the workspace loads.

Don't build everything until we have remote cache set up since some builds take well over an hour.

[attilaolah]

Basic GitHub Actions is now set up.

However, we still don't build the whole repo as it takes a few hours to do so. It should be possible to introduce a nightly run, either on the main branch, or by introducing a staging branch to aggregate pull requests throughout the day, and merge them to main once a day or so, using a fast-forward merge, after a full build.

Either way, the current (basic) CI checks are good enough.