Makefile: clean golangci-lint cache on make clean

To speedup `make lint` we use a local cache.
It seems that there is no sane way to check and just
update the cache, so we'll just provide an option to wipe it.

Using the cache does reduce the time of `make lint` from
1 minute 15sec to 1 sec on my PC. So for consecutive runs,
the cache still makes sense.

Author: schuellerf

Makefile

@@ -180,6 +180,7 @@ clean: db-tests-prune
 	rm -rf $(BUILDDIR)/$(PROCESSED_TEMPLATE_DIR)
 	rm -rf $(GOLANGCI_LINT_CACHE_DIR)
 	rm -rf $(BUILDDIR)/build/
+	rm -rf $(GOLANGCI_LINT_CACHE_DIR)
 	rm -f $(BUILDDIR)/go.local.*
 	rm -f $(BUILDDIR)/container_worker_built.info
 	rm -f $(BUILDDIR)/container_composer_built.info