Make target a build 'artifact' instead of a 'cache'

According to the doc, artifacts can be shared between jobs, while
caches are not designed for that.

.gitlab-ci.yml

 image: docker-registry.univ-nantes.fr/sunye-g/docker-asciidoctor-revealjs
 
-cache:
-  paths:
-  - target
-
 stages:
   - build
   - deploy
@@ -12,6 +8,9 @@ build:
   stage: build
   script:
    - bash /home/compile.sh
+  artifacts:
+    paths:
+      - target
 
 pages:
   stage: deploy