Allow restore tasks to be skipped using SKIP

Previously the only tasks skipped on restore were stored in the backup
manifest. This allows the manifest to be overridden using environment
variables.

Changelog: added

lib/backup/manager.rb

@@ -389,7 +389,8 @@ module Backup
     end
 
     def skipped?(item)
-      backup_information[:skipped] && backup_information[:skipped].include?(item)
+      ENV.fetch('SKIP', '').include?(item) ||
+        backup_information[:skipped] && backup_information[:skipped].include?(item)
     end
 
     def enabled_task?(task_name)

spec/lib/backup/manager_spec.rb

@@ -228,6 +228,20 @@ RSpec.describe Backup::Manager do
       end
     end
 
+    context 'when SKIP env is set' do
+      let(:expected_backup_contents) { %w{backup_information.yml task1.tar.gz} }
+
+      before do
+        stub_env('SKIP', 'task2')
+      end
+
+      it 'executes tar' do
+        subject.create # rubocop:disable Rails/SaveBang
+
+        expect(Kernel).to have_received(:system).with(*tar_cmdline)
+      end
+    end
+
     context 'when the destination is optional' do
       let(:expected_backup_contents) { %w{backup_information.yml task1.tar.gz} }
       let(:definitions) do