gitlab: cache the artefacts required for testing

.gitlab-ci.yml

@@ -12,9 +12,17 @@ build:
     - apt update
     - apt -y install cmake ninja-build python3 ragel
   script: 
+    - mkdir build && cd build
     - cmake -G Ninja -DCMAKE_BUILD_TYPE=Release -DLTO=ON -DTESTS=ON
+    - ninja
+  artifacts:
+    untracked: true
 
 test:
   stage: test
+  before_script:
+    - apt update
+    - apt -y install cmake python3
   script:
+    - cd build
     - ctest -vvv