RuboCop: Use absolute path for cache root directory

This avoids the creation of orphan directories when workspace root is
changed. For example: VSCode Ruby plugin.

.rubocop.yml

@@ -38,7 +38,8 @@ AllCops:
     - 'workhorse/**/*'
     - 'spec/support/*.git/**/*'  # e.g. spec/support/gitlab-git-test.git
     - 'db/ci_migrate/*.rb'      # since the `db/ci_migrate` is a symlinked to `db/migrate`
-  CacheRootDirectory: tmp
+  # Use absolute path to avoid orphan directories with changed workspace root.
+  CacheRootDirectory: <%= Dir.getwd %>/tmp
   MaxFilesInCache: 25000
 
 Cop/AvoidKeywordArgumentsInSidekiqWorkers: