Do not clone the repo when running the review-docs jobs

Previously, we checked out the branch and ran the script from there.
However, when the MR got merged and the cleanup job was run it would
fail since it could not find the now deleted branch.

This commit rather downloads the script from GitLab and skips
downloading the artifacts/dependencies/cache to speed up the process.

.gitlab-ci.yml

@@ -183,11 +183,20 @@ build-package:
   image: ruby:2.4-alpine
   before_script:
     - gem install gitlab --no-doc
+    # We need to download the script rather than clone the repo since the
+    # review-docs-cleanup job will not be able to run when the branch gets
+    # deleted (when merging the MR).
+    - apk add --update openssl
+    - wget https://gitlab.com/gitlab-org/gitlab-ce/raw/master/scripts/trigger-build-docs
+    - chmod 755 trigger-build-docs
   services: []
+  cache: {}
+  dependencies: []
+  artifacts: {}
   variables:
     SETUP_DB: "false"
     USE_BUNDLE_INSTALL: "false"
-  cache: {}
+    GIT_STRATEGY: none
   when: manual
   only:
     - branches
@@ -204,7 +213,7 @@ review-docs-deploy:
     url: http://preview-$CI_COMMIT_REF_SLUG.$DOCS_REVIEW_APPS_DOMAIN/$DOCS_GITLAB_REPO_SUFFIX
     on_stop: review-docs-cleanup
   script:
-    - scripts/trigger-build-docs deploy
+    - ./trigger-build-docs deploy
 
 # Cleanup remote environment of gitlab-docs
 review-docs-cleanup:
@@ -214,7 +223,7 @@ review-docs-cleanup:
     name: review-docs/$CI_COMMIT_REF_NAME
     action: stop
   script:
-    - scripts/trigger-build-docs cleanup
+    - ./trigger-build-docs cleanup
 
 # Retrieve knapsack and rspec_flaky reports
 retrieve-tests-metadata: