Fix CI caching for Gradle jobs

Because Gradle jobs are run in the directory of the sub project, using a relative path for GRADLE_USER_HOME results in the use of separate caches. Therefore, an absolute path is now set.

For previously run jobs, GitLab reported:

WARNING: .gradle-user-home/caches: no matching files. Ensure that the artifact path is relative to the working directory (/builds/particify/dev/foss/arsnova-server)

Now it once again reports the number of files for caching:

.gradle-user-home/caches: found 4595 matching artifact files and directories

Regression-MR: !593 (merged)