Set $COMMIT_NOTES_URL within `script:`

Doing it in `before_script:` would
override the extended `before_script:` in `.notify`

.gitlab/ci/notifications.gitlab-ci.yml

@@ -10,8 +10,8 @@ schedule:package-and-qa:notify-success:
   extends:
     - .only-canonical-schedules
     - .notify
-  before_script:
-    - export COMMIT_NOTES_URL="https://$CI_SERVER_HOST/$CI_PROJECT_PATH/commit/$CI_COMMIT_SHA#notes-list"
+  variables:
+    COMMIT_NOTES_URL: "https://$CI_SERVER_HOST/$CI_PROJECT_PATH/commit/$CI_COMMIT_SHA#notes-list"
   script:
     - 'scripts/notify-slack qa-master ":tada: Scheduled QA against master passed! :tada: See $CI_PIPELINE_URL. For downstream pipelines, see $COMMIT_NOTES_URL" ci_passing'
   needs: ["schedule:package-and-qa"]
@@ -21,8 +21,8 @@ schedule:package-and-qa:notify-failure:
   extends:
     - .only-canonical-schedules
     - .notify
-  before_script:
-    - export COMMIT_NOTES_URL="https://$CI_SERVER_HOST/$CI_PROJECT_PATH/commit/$CI_COMMIT_SHA#notes-list"
+  variables:
+    COMMIT_NOTES_URL: "https://$CI_SERVER_HOST/$CI_PROJECT_PATH/commit/$CI_COMMIT_SHA#notes-list"
   script:
     - 'scripts/notify-slack qa-master ":skull_and_crossbones: Scheduled QA against master failed! :skull_and_crossbones: See $CI_PIPELINE_URL. For downstream pipelines, see $COMMIT_NOTES_URL" ci_failing'
   needs: ["schedule:package-and-qa"]