CI: Set persist-credentials to true to allow pushing to the gh-pages branch

[seisman]

The "Docs" workflow failed to upload the built HTML files to the gh-pages branch since PR #3861.

The git push -fq origin gh-pages command failed with errors like below (https://github.com/GenericMappingTools/pygmt/actions/runs/13937126371/job/39007072378#step:14:4408):

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

but the workflow step doesn't fail. That's why we didn't catch it in PR #3861.

The command fails because we set persist-credentials to false in that PR. This PR fixes the issue.

[weiji14]

but the workflow step doesn't fail. That's why we didn't catch it in PR #3861.

Good catch, I knew there had to be at least one job that required extra permissions 😆

[seisman]

Hmm, it still doesn't work, but now with different error (https://github.com/GenericMappingTools/pygmt/actions/runs/14044608036/job/39322435789#step:14:4472):

Pushing changes to gh-pages.
remote: Permission to GenericMappingTools/pygmt.git denied to github-actions[bot].
fatal: unable to access 'https://github.com/GenericMappingTools/pygmt/': The requested URL returned error: 403

[seisman]

It's likely due to the permissions: {} setting added in PR #3861, which disables all permissions for GITHUB_TOKEN.

[seisman]

The slash command also no longer works (#3866 (comment))

https://github.com/GenericMappingTools/pygmt/actions/runs/14049785898/job/39337881112?pr=3866