fix(runners): fix type mismatch in pool role conditional

The conditional for the runner role in pool.tf returned inconsistent types:
the true branch produced an object with a single 'arn' attribute while the
false branch returned the full aws_iam_role resource (16 attributes).

Terraform requires both branches to have consistent types. Move the
conditional inside the object so both branches produce a string for the
'arn' key.

Fixes type error introduced in github-aws-runners#4875.

Upstream-PR: github-aws-runners#5156

Author: Brend-Smits

modules/runners/pool.tf

@@ -51,7 +51,7 @@ module "pool" {
       group_name                           = var.runner_group_name
       name_prefix                          = var.runner_name_prefix
       pool_owner                           = var.pool_runner_owner
-      role                                 = var.iam_overrides["override_runner_role"] ? { arn = var.iam_overrides["runner_role_arn"] } : aws_iam_role.runner[0]
+      role                                 = { arn = var.iam_overrides["override_runner_role"] ? var.iam_overrides["runner_role_arn"] : aws_iam_role.runner[0].arn }
       use_dedicated_host                   = var.use_dedicated_host
     }
     subnet_ids                           = var.subnet_ids