Fix release push: disable persist-credentials when app token is used

[arcivanov]

Summary

• Upgrade checkout from actions/checkout@v5 to @v6
• Set persist-credentials to false when release-app-id is provided, preventing the default GITHUB_TOKEN credential helper from overriding the app token during git push
• When no app is configured, credentials persist as before (no behavior change)

Fixes release pushes failing with GH013: Repository rule violations on repos with branch protection rulesets.

Test plan

• Release push succeeds on a repo with branch protection and PyBuilder app configured
• Non-release builds without app credentials are unaffected