Set envvar for redis-cluster-client gem in gstg

What does this MR do?

This MR pre-sets the environment variable as part of the redis gem upgrades (gitlab-com/gl-infra/scalability#2727 (closed)) to tune the redis-cluster-client start-up behaviour.

More details regarding the envvar is in gitlab-com/gl-infra/scalability#2571 (comment 1632753362)

Author Check-list

Please read the Contributing document and once you do, complete the following:

• Check if all of the following apply:
  • Assign to the correct reviewer per the contributing document
  • Apply the correct metadata per the contributing document
  • Link to related MRs for applying the changes on other environments
  • Link to related Chef changes
  • If necessary link to a Criticality 4 Change Request issue

Reviewer Check-list

• Check if all of the following apply:
  • Reviewed the diff jobs to confirm changes are as expected
  • No changes shown in the diffs not associated with this MR - This may require a rebase or further investigation

Applier Check-list

• Make sure there is no ongoing deployment for the affected envs before merging (see #announcements slack channel)