Replace ☢️ with :danger: in Buildkite pipeline (#690)

Author: iangmaia

.buildkite/pipeline.yml

@@ -61,7 +61,7 @@ steps:
 
   - group: "Linters"
     steps:
-      - label: ☢️ Danger - PR Check
+      - label: ":danger: Danger - PR Check"
         command: danger
         key: danger
         if: build.pull_request.id != null