Backport of E2E: use a variable for region into release/1.9.x

[hc-github-team-nomad-core]

Backport

This PR is auto-generated from #24693 to be assessed for backporting due to the inclusion of the label backport/1.9.x.

🚨

Warning automatic cherry-pick of commits failed. If the first commit failed,
you will see a blank no-op commit below. If at least one commit succeeded, you
will see the cherry-picked commits up to, not including, the commit where
the merge conflict occurred.

The person who merged in the original PR is:
@tgross
This person should manually cherry-pick the original PR into a new backport PR,
and close this one when the manual backport PR is merged in.

merge conflict error: POST https://api.github.com/repos/hashicorp/nomad/merges: 409 Merge conflict []

The below text is copied from the body of the original PR.

---

In #24644 we set the region to "e2e" but forgot to setup the TLS certificate names appropriately. Swap the region out for a variable instead.

---

Overview of commits

• abeae5c

[hashicorp-cla-app]

All committers have signed the CLA.

[hashicorp-cla-app]

Thank you for your submission! We require that all contributors sign our Contributor License Agreement ("CLA") before we can accept the contribution. Read and sign the agreement

Learn more about why HashiCorp requires a CLA and what the CLA includes

---

temp seems not to be a GitHub user.
You need a GitHub account to be able to sign the CLA.
If you have already a GitHub account, please add the email address used for this commit to your account.

_{Have you signed the CLA already but the status is still pending? Recheck it.}

[tgross]

Manually fixed.