Add a core-convert job

Closes #3414168

.gitlab-ci.yml

@@ -126,7 +126,7 @@ core-convert:
     - cp -R scripts $MODULE_DIR
     # Strip the `-dev` suffix out of $_TARGET_CORE to reveal the name of the Drupal core branch.
     - CORE_BRANCH=`echo $_TARGET_CORE | sed -e "s/-dev$//"`
-    - composer run core-convert --working-dir=$MODULE_DIR -- $_WEB_ROOT --gitlabci --core_target_branch=$CORE_BRANCH
+    - composer run core-convert --working-dir=$MODULE_DIR -- $CI_PROJECT_DIR/$_WEB_ROOT --gitlabci --core_target_branch=$CORE_BRANCH
 
 phpcs:
   before_script: