ci(hacktoberfest_prep): authenticate git push via gh credential helper

[priya-sundaram-dev]

Describe your change

Fixes the hacktoberfest_prep scheduled/manual workflow, which has been failing on the "Open or update the tracker pull request" step (example run):

fatal: could not read Username for 'https://github.com': No such device or address
##[error]Process completed with exit code 128.

Root cause: the actions/checkout step runs with persist-credentials: false, so git is left with no stored token. The step then calls plain git push, which has nothing to authenticate with. GH_TOKEN is only wired into the gh CLI, not into git itself, so gh pr create would work but the git push before it never gets that far.

Fix: run gh auth setup-git before the push. That registers the already-bundled gh as git's credential helper (credential.https://github.com.helper = !gh auth git-credential), so git push reuses GH_TOKEN. No third-party action, and — unlike embedding the token in the remote URL — nothing secret is written into git config.

Keeping persist-credentials: false is deliberate (it's the safer default zizmor recommends); this just restores auth for the one push that needs it.

• Add an algorithm?
• Fix a bug or typo in an existing algorithm? — CI workflow fix
• Add or change tests?
• Documentation change?