Enable caching for JAliEn-ROOT source directory

This is required to support rerunning just the test phase of the pipeline. The JAliEn-ROOT repo (with headers) is fetched in the build stage, it's built and library is stored in sw which is preserved. In the test phase, we need the compiled library and headers, but the headers may not be present if not reused from cache. It's a ducktape solution, but it should work for now.