Make API key env var Slack specific

Author: spastorelli

integrations/slack/gitbook-manifest.yaml

@@ -78,5 +78,5 @@ secrets:
     CLIENT_ID: ${{ env.SLACK_CLIENT_ID }}
     CLIENT_SECRET: ${{ env.SLACK_CLIENT_SECRET }}
     SIGNING_SECRET: ${{ env.SLACK_SIGNING_SECRET }}
-    OPENAI_API_KEY: ${{ env.OPENAI_API_KEY }}
+    OPENAI_API_KEY: ${{ env.SLACK_OPENAI_API_KEY }}
 target: space