fix(ci): retain backend digests for release retries (#12160)

The v4.10.0 ace-step and VibeVoice merge jobs started just after their
digest artifacts expired. Keep the small digest artifacts for seven
days so a multi-day release matrix can finish publishing its images.

Assisted-by: Codex:gpt-6

Co-authored-by: localai-org-maint-bot <306269227+localai-org-maint-bot@users.noreply.github.com>

.github/workflows/backend_build.yml

@@ -252,7 +252,8 @@ jobs:
           name: digests${{ inputs.tag-suffix }}--${{ inputs.platform-tag || 'single' }}
           path: /tmp/digests/*
           if-no-files-found: error
-          retention-days: 1
+          # Release matrices and their retries can outlive a one-day artifact.
+          retention-days: 7
 
       - name: Build (PR)
         uses: docker/build-push-action@v7