ci: make slack notification non-blocking when secret is missing

.woodpecker/api.yml

@@ -188,6 +188,7 @@ steps:
 
   - name: slack
     image: woodpeckerci/plugin-slack
+    failure: ignore
     settings:
       webhook:
         from_secret: slack-webhook-url

.woodpecker/frontend.yml

@@ -152,6 +152,7 @@ steps:
 
   - name: slack
     image: woodpeckerci/plugin-slack
+    failure: ignore
     settings:
       webhook:
         from_secret: slack-webhook-url