ci: fix server image publish to use repo-scoped package name

Use ${{ github.repository }}/server as the GHCR image name so the
GITHUB_TOKEN has write access. The previous hardcoded name pointed at
the archived repo's package namespace.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: Mazyod

.github/workflows/publish-server-image.yml

@@ -8,7 +8,7 @@ on:
 
 env:
   REGISTRY: ghcr.io
-  IMAGE_NAME: mazyod/phoenix-integration-tester
+  IMAGE_NAME: ${{ github.repository }}/server
 
 jobs:
   publish: