Stub ENV in the backup task spec file

lib/backup/manager.rb

@@ -101,8 +101,7 @@ module Backup
     end
 
     def unpack
-      Dir.chdir(backup_path)
-
+      Dir.chdir(backup_path) do
         # check for existing backups in the backup dir
         if backup_file_list.empty?
           $progress.puts "No backups found in #{backup_path}"
@@ -147,6 +146,7 @@ module Backup
           exit 1
         end
       end
+    end
 
     def tar_version
       tar_version, _ = Gitlab::Popen.popen(%w(tar --version))

spec/lib/gitlab/backup/manager_spec.rb

@@ -172,10 +172,6 @@ describe Backup::Manager do
   end
 
   describe '#unpack' do
-    before do
-      allow(Dir).to receive(:chdir)
-    end
-
     context 'when there are no backup files in the directory' do
       before do
         allow(Dir).to receive(:glob).and_return([])