Actions: Fix build for PRs (Coveralls)

The Java 8 build doesn't work for PRs for some reason I gave up on... IIRC because of coveralls. Would be nice for the Java 8 build to work.

~~At the same time, it would probably be good to add Java 17 as build, too.~~ Skipping for now as I want to stay on JDK 8 for a little longer. Probably not much added value in testing a lot of newer versions.