Add "cleanup_file_variables" sub stage

GitLab Runner 13.5.0 added the "cleanup_file_variables" sub stage to the
"run_exec" stage of every GitLab CI job.  Update all Custom executor
scripts so that they handle that sub stage appropriately.
3 jobs for mnowak/add-cleanup_file_variables-stage in 9 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Test
passed #1405392
linux docker amd64
flake8

00:00:09

passed #1405393
linux docker amd64
pylint

00:00:09

passed #1405394
linux docker amd64
shellcheck

00:00:08