Partly revert "ENH: Run testenv always."

This reverts commit 78d636e4.

.gitlab-ci.yml

@@ -191,6 +191,10 @@ build-testenv:
   image: docker:20.10
   stage: setup
   timeout: 2h
+  only:
+    - schedules
+    - web
+    - pushes
   needs: []
   script:
       - df -h