Improve caching logic

Makefile

-.PHONY: build
-$(REPO_PATH)/index.jar: $(REPO_PATH)
+$(REPO_PATH)/index.jar: s3
 	fdroid update --use-date-from-apk
 	fdroid server update
 
-$(REPO_PATH): s3
-	rsync -rlpcgoDv $(S3_PATH)/*-release.apk $(REPO_PATH)/
+s3: update_s3
 
-.PHONY: s3
-s3:
+.PHONY: update_s3
+update_s3:
 	s3cmd sync --preserve s3://releases/quasseldroid-ng/ $(S3_PATH)/
+	rsync -rlpcgoDv $(S3_PATH)/*-release.apk $(REPO_PATH)/