Add sourcebot cloud environment prop to staging workflow

Author: brendan-kellam

.github/workflows/staging-ghcr-public.yml

@@ -71,6 +71,7 @@ jobs:
           build-args: |
             SOURCEBOT_VERSION=${{ github.ref_name }}
             POSTHOG_PAPIK=${{ secrets.POSTHOG_PAPIK }}
+            NEXT_PUBLIC_SOURCEBOT_CLOUD_ENVIRONMENT=staging
 
       - name: Export digest
         run: |