test(lago): give the skipped case a reason

The lint forbids a bare `--- SKIP`, which is the right rule: a bypass
without a stated reason is indistinguishable from an accident.

Author: AlinsRan

t/plugin/lago.t

@@ -79,4 +79,4 @@ cd t && pnpm test plugin/lago.spec.mts 2>&1
 failed to execute the script with status
 --- response_body eval
 qr/PASS plugin\/lago.spec.mts/
---- SKIP
+--- SKIP: needs a live Lago stack cloned and started from the network (#12904)