CI: Add a new workflow to cache dvc cache

[seisman]

For new contributors, it's frustrating to see that so mamy workflows fail due to the DVC issue reported in #4147.

This PR adds a workflow for this issue. It adds a new workflow cache_dvc.yaml which pulls the baseline images from DagsHub and uploads the .dvc/cache file as GitHub artifacts.

For PRs from forks, the dvc pull command fails, so we can download the dvc cache from the GitHub artifact.

Below are workflows that use dvc pull and may fail:

• ci_tests_dev.yaml
• ci_tests_dev.yaml
• dvc-diff.yaml: The cache won't work in this workflow, since the dvc cache reflect baseline images in the main branch, but this workflow needs baseline images in both the main and the current branch. But it's unlikely that PRs from new contributors will trigger this workflow, since they can't do 'dvc push' either. So no need to apply the workaround
• release-baseline-images.yml: Doesn't run in PRs, so no need to apply the workaround

Tests

• The workflow run succeeded and the .dvc/cache directory was uploaded https://github.com/GenericMappingTools/pygmt/actions/runs/19008233397?pr=4184
• The "Download DVC cache as artifacts from GitHub" step is skipped when dvc pull command succeeds (https://github.com/GenericMappingTools/pygmt/actions/runs/19008233364/job/54285305945?pr=4184)
• In commit 6d9be57 (with a fix in bd4b348), I made changes to make the "DVC pull" step fail with an exit code of 1. As you can see, the workflows downloads the cache from GitHub artifact and then the workflows pass.

[seisman]

Reference: https://docs.github.com/en/actions/reference/workflows-and-actions/contexts#steps-context

[weiji14]

Thank you! Just a couple of suggestions.

[weiji14]

Could probably this as a separate job under .github/workflows/cache_data.yaml if want to reduce the number of workflow files.

[seisman]

These two jobs have different trigger conditions. The cache_data workflow is usually scheduled to run weekly but the cache_dvc workflow needs to run when .dvc files change.

[weiji14]

Ah yes, ok to keep them separate then.