CI: Add Yarn.
diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index 3f0dba2..523e341 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -6,6 +6,8 @@
 verify:
   stage: check
   script:
+    - apt-get update
+    - apt-get -y --no-install-recommends yarn
     - mvn verify
 
 sast: