re-add GHA job that builds ROOT

[KrisThielemans]

#901 installed ROOT from binary download, as opposed to building it, due to problems with the latter see #899. That means we no longer test building ROOT ourselves, except in the docker builds. It'd be good to have a job that does the test explicitly. It then probably should be marked to allow failure, see #898. This is low priority though, until we download ROOT in the docker build as well.