gitlab-ci.yml: Install yarn for new F/E building system

.gitlab-ci.yml

@@ -37,7 +37,7 @@ build-src:
     - curl -sL https://deb.nodesource.com/setup_10.x | bash -
     - apt update
     - apt install -y nodejs
-    - npm install -g html-minifier@3.5.7 uglify-js@3.4.1 sass@1.5.1
+    - npm install -g yarn@1.10.0
   script:
     - cd /go/src/$CI_PROJECT_NAME
     - make devel