GHA: Building the module list fails when adding new modules

[lenaschoenburg]

As seen here: #9258

When a new module was added and introduced as a dependency, maven refuses to return the list of modules because it needs to resolve the dependency but can't find it in the maven repository.

To fix this we could simply mvn install before asking maven for the module list, but since this issue (missing mvn install) has come up a number of times and it would slow down the entire pipeline by an additional minute or so, we can also look at other solutions:

• Just run mvn install first
• Introduce a new job that runs mvn install once and makes the built artifacts available to all other jobs, see [SPIKE] [WIP] Refactor Elasticsearch Exporter tests #9258 (comment)
• Don't ask maven for a list of modules, see [SPIKE] [WIP] Refactor Elasticsearch Exporter tests #9258 (comment)