CI: tweak cache.key to fix error

Error is:

  jobs:test:cache:key config cannot contain the "/" character

.gitlab-ci.yml

@@ -18,7 +18,7 @@ test:
   after_script:
   - rm gitID.txt
   cache:
-    key: "$CI_PROJECT_NAMESPACE/$CI_PROJECT_NAME/$CI_JOB_NAME"
+    key: "$CI_PROJECT_NAMESPACE:$CI_PROJECT_NAME:$CI_JOB_NAME"
     untracked: true
   artifacts:
     expire_in: 4w