Do not use ccache in Gitlab-CI jobs
After the last few accepted MRs (or a change in Gitlab?) the CI job always fails in the debug builds because it cannot upload the artifacts.
We already had a similar problem which forced me to reduce the cache size, making it pointless for debug builds (not fitting anymore). While the optimized builds were profiting from the ccache cache, the whole pipeline was anyway delayed by the debug builds.
With this MR I stop using ccache, practically fixing the problem with debug builds.