Update workflows

[DarkaMaul]

This PR contains two workflow improvements:

1. Removes redundant default setting (per @di's feedback in Use zizmor from PyPI #17512 (comment))
2. Optimizes CI performance by running zizmor checks only when .github/workflows/** files change

Note: While change #2 reduces CI load, new zizmor checks won't run until either:

• Workflow files are modified
• Weekly scheduled run occurs (Monday)

I am happy to split these changes into separate PRs if preferred.

/cc @woodruffw

[di]

Note: While change #2 reduces CI load, new zizmor checks won't run until either

FWIW I don't think CI load has been an issue here. I assume this also mean that we won't run new versions of zizmor until we modify workflows / the cron runs as well, correct? I think I'm OK with keeping the workflow as-is to ensure we're up to date.

[woodruffw]

Thanks @DarkaMaul!