🔧(dns) make target zone for communes domains configurable

Add a configuration setting tied to an env var, so we can have
a separate zone for staging/preprod.

src/frontend/apps/e2e/__tests__/app-desk/domain-provisioning.spec.ts

@@ -23,7 +23,7 @@ test.describe('When a commune, domain is created on first login via ProConnect',
     await expect(
       page.getByLabel('Mail domains panel', { exact: true }),
     ).toBeVisible();
-    await expect(page.getByText('merlaut.collectivite.fr')).toHaveCount(1);
+    await expect(page.getByText('merlaut.test.collectivite.fr')).toHaveCount(1);
     await expect(page.getByText('No domains exist.')).toHaveCount(0);
   });
 });