CI: Set persist-credentials: true in format-command.yml

[weiji14]

Description of proposed changes

Needed to fix fatal: could not read Username for 'https://github.com': No such device or address.

Address #4136 (comment), patches #3874 and #3861

Preview:

Reminders

• Run make format and make check to make sure the code follows the style guide.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Add new public functions/methods/classes to doc/api/index.rst.
• Write detailed docstrings for all functions/methods.
• If wrapping a new module, open a 'Wrap new GMT module' issue and submit reasonably-sized PRs.
• If adding new functionality, add an example to docstrings or tutorials.

Slash Commands

You can write slash commands (/command) in the first line of a comment to perform
specific operations. Supported slash command is:

• /format: automatically format and lint the code

[weiji14]

Thanks @seisman, confirmed to be working now, see trigger after comment at #4134 (comment) 🎉