Make CI builds resilient against disconnects on GH Actions infrastructure

Quite often, there are build failures on GH Actions, caused by some networking hick-up when obtaining dependencies from Central, e.g. here:

Error: Plugin org.apache.maven.plugins:maven-failsafe-plugin:3.0.0-M3 or one of its dependencies could not be resolved: Failed to read artifact descriptor for org.apache.maven.plugins:maven-failsafe-plugin:jar:3.0.0-M3: Could not transfer artifact org.apache.maven.plugins:maven-failsafe-plugin:pom:3.0.0-M3 from/to central (https://repo.maven.apache.org/maven2): Transfer failed for https://repo.maven.apache.org/maven2/org/apache/maven/plugins/maven-failsafe-plugin/3.0.0-M3/maven-failsafe-plugin-3.0.0-M3.pom: Connection timed out (Read failed) -> [Help 1]

We should be able to somewhat mitigate this by using the GH Actions caching facility to cache a Maven repo with the external dependencies (and only those, i.e. not our own build artifacts). To create the cache, mvn dependency:go-offline could be run once.

Update: it seems we can address the issue by passing the right options to Maven (see my second comment), avoiding those disconnects. Let's investigate separately why the caching which is in place already still doesn't avoid the repeated downloads of those plug-ins.