Add and apply ShadowingOuterLocalVariable rule

Naming two variables the same way is confusing at the very least, and
can lead to hard to debug errors. That's why the Ruby interpreter issues
a warning when we do so.

app/controllers/tracking/budget_investments_controller.rb

@@ -58,14 +58,13 @@ class Tracking::BudgetInvestmentsController < Tracking::BaseController
                               }
                             ]
 
-      filters = investment_headings.inject(all_headings_filter) do |filters, heading|
+      investment_headings.inject(all_headings_filter) do |filters, heading|
         filters << {
           name: heading.name,
           id: heading.id,
           count: investments.select { |i| i.heading_id == heading.id }.size
         }
-      end
-      filters.uniq
+      end.uniq
     end
 
     def restrict_access_to_assigned_items