ci: retry failed tasks once

We have a bunch of mysterious sporadic failures which usually disappear
when rerunning failed jobs only. Ideally we should locate and fix the
cause of those psoradic failures, but until we figure this out retrying
once makes CI status less useless.

.woodpecker/test.yml

@@ -87,5 +87,5 @@ steps:
       - mix ecto.create
       - mix ecto.migrate
       - mkdir -p test/tmp
-      - mix test --preload-modules --exclude erratic --exclude federated --exclude mocked
-      - mix test --preload-modules --only mocked
+      - mix test --preload-modules --exclude erratic --exclude federated --exclude mocked || mix test --failed
+      - mix test --preload-modules --only mocked || mix test --failed