use staging posthog papik when building staging image

Author: msukkari

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

@@ -70,7 +70,7 @@ jobs:
           outputs: type=image,name=${{ env.REGISTRY_IMAGE }},push-by-digest=true,name-canonical=true,push=true
           build-args: |
             SOURCEBOT_VERSION=${{ github.ref_name }}
-            POSTHOG_PAPIK=${{ secrets.POSTHOG_PAPIK }}
+            POSTHOG_PAPIK=${{ secrets.STAGING_POSTHOG_PAPIK }}
 
       - name: Export digest
         run: |