Adds posthog api key as playbook variable

Author: jagabomb

.github/workflows/pr_env.yaml

@@ -102,6 +102,7 @@ jobs:
             nginx_ssl_key_as_base64='true' \
             nginx_ssl_cert='${{ secrets.PR_ENV_SSL_CERT }}' \
             nginx_ssl_key='${{ secrets.PR_ENV_SSL_CERT_KEY }}' \
+            posthog_api_key='${{ secrets.POSTHOG_API_KEY_DEMO }}' \
             install_method='docker'"
 
   run-photofinish-demo-env: