Do not run prepare-env job on push

It's useless now, and confusing as it will reset the pipeline status to success.

.gitlab-ci.yml

@@ -12,6 +12,8 @@ prepare-env:
   artifacts:
     reports:
       dotenv: prepare.env
+  rules:
+    - if: $CI_PIPELINE_SOURCE == "schedule"
 
 build-deps-debug:
   stage: deps