e2e: Ensure UI is enabled. (#25620)

The `ui.enabled` parameter is a non-pointer bool which means the
merge function is unable to differentiate between false and not
set. When e2e introduced the `ui.show_cli_hints` configuration
parameter, the way we merge meant the UI became disabled.

e2e/terraform/provision-infra/provision-nomad/etc/nomad.d/base.hcl

@@ -30,5 +30,6 @@ telemetry {
 }
 
 ui {
+  enabled        = true
   show_cli_hints = false
 }