Run slack dep installation as post-step in CI

.circleci/config.yml

@@ -9,6 +9,7 @@ workflows:
           context: server-slack-notifications
           dir: .
           post-steps:
+            - install-slack-deps
             - slack-notify-on-failure
 
 commands:
@@ -18,7 +19,6 @@ commands:
           command: apk add curl jq
   slack-notify-on-failure:
     steps:
-      - install-slack-deps
       - slack/notify:
           event: fail
           template: basic_fail_1