fix: add event filtering for slack-on-failure

Author: ivictbor

live-demo/deploy/.woodpecker.yml

@@ -25,8 +25,10 @@ steps:
       - NPM_TOKEN
       - SLACK_WEBHOOK 
 
-  slack-after-build:
+  slack-on-failure:
     image: plugins/slack
+    when:
+      - event: push
     secrets:
       - DEVELOPERS_SLACK_WEBHOOK
     webhook: $DEVELOPERS_SLACK_WEBHOOK