Fixed flicker of download project button

Closes #14177

app/views/projects/buttons/_download.html.haml

 - unless @project.empty_repo?
   - if can? current_user, :download_code, @project
-    = link_to archive_namespace_project_repository_path(@project.namespace, @project, ref: @ref, format: 'zip'), class: 'btn has_tooltip', rel: 'nofollow', title: "Download ZIP" do
+    = link_to archive_namespace_project_repository_path(@project.namespace, @project, ref: @ref, format: 'zip'), class: 'btn has_tooltip', data: {container: "body"}, rel: 'nofollow', title: "Download ZIP" do
       = icon('download')