CI: Use matrix.os instead of runner.os in artifact name

[weiji14]

Description of proposed changes

Fixes error Failed to CreateArtifact: Received non-retryable error: Failed request: (409) Conflict: an artifact with this name already exists on the workflow run on the ubuntu-24.04-arm job. Sample job failure at https://github.com/GenericMappingTools/pygmt/actions/runs/13231430400/job/36929163034#step:16:17

Patches #3778

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

[seisman]

Looks good.