Avoid join in PipelinesFinder#by_username

When we move `ci_*` tables to a separate database this join will not be
possible. This was extracted from
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/62092 .

app/finders/ci/pipelines_finder.rb

@@ -119,11 +119,12 @@ module Ci
 
     # rubocop: disable CodeReuse/ActiveRecord
     def by_username(items)
-      if params[:username].present?
-        items.joins(:user).where(users: { username: params[:username] })
-      else
-        items
-      end
+      return items unless params[:username].present?
+
+      user_id = User.by_username(params[:username]).pluck_primary_key.first
+      return Ci::Pipeline.none unless user_id
+
+      items.where(user_id: user_id)
     end
     # rubocop: enable CodeReuse/ActiveRecord