Allow 'source' as an index param for pipelines if FF is enabled

This was missed during a conflict-resolve for
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/67964.
Without this, pipeines can not be filtered by source from the UI.
Signed-off-by: Balasankar "Balu" C <balasankar@gitlab.com>

app/controllers/projects/pipelines_controller.rb

@@ -297,7 +297,10 @@ class Projects::PipelinesController < Projects::ApplicationController
   end
 
   def index_params
-    params.permit(:scope, :username, :ref, :status)
+    permitted_params = [:scope, :username, :ref, :status]
+    permitted_params << :source if Feature.enabled?(:pipeline_source_filter, project, default_enabled: :yaml)
+
+    params.permit(*permitted_params)
   end
 
   def enable_code_quality_walkthrough_experiment