fix spec and added changelog

changelogs/unreleased/fix-backup-restore-resume.yml

+---
+title: Make backup task to continue on corrupt repositories
+merge_request: 11962
+author:

spec/lib/gitlab/backup/repository_spec.rb

 require 'spec_helper'
 
 describe Backup::Repository, lib: true do
-  include StubENV
-
   let(:progress) { StringIO.new }
   let!(:project) { create(:empty_project) }