Clone repo in `build-assets-image`

When this job runs in private forks such as
`dev.gitlab.org/gitlab/gitlab-ee`, the raw files
are not publicly available for download

.gitlab/ci/frontend.gitlab-ci.yml

@@ -66,16 +66,11 @@ build-assets-image:
     - .frontend:rules:gitlab-assets-compile-pull-cache
   stage: build-images
   needs: ["gitlab:assets:compile pull-cache"]
-  variables:
-    GIT_STRATEGY: none
   script:
-    - wget -O ./build_assets_image "${CI_PROJECT_URL}/raw/${CI_COMMIT_SHA}/scripts/build_assets_image"
-    - wget -O ./Dockerfile.assets "${CI_PROJECT_URL}/raw/${CI_COMMIT_SHA}/Dockerfile.assets"
-    - chmod +x build_assets_image
     # TODO: Change the image tag to be the MD5 of assets files and skip image building if the image exists
     # We'll also need to pass GITLAB_ASSETS_TAG to the trigerred omnibus-gitlab pipeline similarly to how we do it for trigerred CNG pipelines
     # https://gitlab.com/gitlab-org/gitlab/issues/208389
-    - ./build_assets_image
+    - scripts/build_assets_image
 
 .compile-assets-metadata:
   extends: