Add conditional for Atmos installation step

[jamengual]

This pull request introduces a small conditional improvement to the workflow configuration. The change ensures that the Atmos installation step only runs when an Atmos version is explicitly provided.

[github-actions]

Changes Found for foobar/changes in plat-ue2-sandbox

Plan: 1 to add, 0 to change, 0 to destroy.

To reproduce this locally, run:

atmos terraform plan foobar/changes -s plat-ue2-sandbox

---

Create

+ random_id.foo[0]

Terraform Plan Summary

  # random_id.foo[0] will be created
  + resource "random_id" "foo" {
      + b64_std     = (known after apply)
      + b64_url     = (known after apply)
      + byte_length = 8
      + dec         = (known after apply)
      + hex         = (known after apply)
      + id          = (known after apply)
      + keepers     = {
          + "seed" = "foo-plat-ue2-sandbox-blue"
        }
    }

Plan: 1 to add, 0 to change, 0 to destroy.

Warning: Test warning summary

  with data.validation_warning.warn[0],
  on main.tf line 20, in data "validation_warning" "warn":
  20: data "validation_warning" "warn" {

Test warning details

Warning

Warning: Test warning summary

  with data.validation_warning.warn[0],
  on main.tf line 20, in data "validation_warning" "warn":
  20: data "validation_warning" "warn" {

Test warning details

[github-actions]

These changes were released in v5.5.0.