Group output if running under GitHub Actions

[javierhonduco]

A small but very useful thing to do when the output in CI systems is very large is to group parts of the output in sections that can be unfolded by the user. In GitHub Actions this can be accomplished by surrounding the output with ::group:: and ::endgroup:: (see the docs). The GITHUB_ACTIONS environment variable is set if running in this CI system.

If nobody implements this in the next few days and Daniel is happy with this change I will put a PR when I have some spare cycles 😄 .