Handle manually the checkout step

Currently, `git lfs checkout` is much more efficient to download LFS data
than `git checkout` (50s / 4min).

.gitlab-ci.yml

 variables:
   DOCKER_DRIVER: overlay2
   GIT_DEPTH: "3"
+  GIT_CHECKOUT: "false"
+
+# Git checkout is disabled to let us handle efficiently the Git-LFS
+before_script:
+  - git lfs fetch origin $CI_COMMIT_REF_NAME
+  - git checkout $CI_COMMIT_REF_NAME
 
 stages:
   - build