gitlab-ci: do not install dependencies for pages

All the job does is copy the artifact to public directory.

.gitlab-ci.yml

@@ -12,7 +12,7 @@ cache:
   paths:
     - .venv/
 
-before_script:
+.setup_build: &setup_build
   - apt-get update
   - apt-get dist-upgrade -y
   - >
@@ -33,8 +33,6 @@ before_script:
     sed
   # Convenient hack until this is fixed.
   - ln -s /usr/bin/python3 /usr/bin/python
-
-.setup_build: &setup_build
   - python -m venv .venv
   - . ./.venv/bin/activate
   - python -m pip install -r requirements.txt