CI: pin docs workflow to Pixi v0.74.0 to unblock locked install

[Copilot]

The Docs workflow build job started failing at pixi install --locked due to lockfile/workspace identity mismatch (notably on GSAS-II source package resolution) under Pixi v0.76.2. This PR aligns the workflow runtime with the lockfile-compatible Pixi version.

• Workflow/tooling alignment

  • Update .github/workflows/docs.yml to use Pixi v0.74.0 in build.
  • No changes to package manifests or lockfile contents; scope is CI workflow only.

• Failure mechanism addressed

  • Prevents lock file not up-to-date with the workspace from aborting the docs build before Sphinx execution.

# .github/workflows/docs.yml
- uses: prefix-dev/setup-pixi@v0.9.4
  with:
    pixi-version: v0.74.0

[Adam Corrao (AdamCorrao)]

I was able to reproduce a successful local smoke test with Pixi 0.71.3 (pixi install --locked, pixi run docs, and docs/_build/html/index.html all succeeded). That suggests the issue is with newer Pixi behavior rather than 0.74.0 specifically. Not clear why earlier pixi v7 versions such as 0.74 work, but 0.76.2 was problematic - perhaps it was caused by using a newer version to generate the lockfile.

[Adam Corrao (AdamCorrao)]

Fixed mismatch of pixi versions seems to have resolved docs build.

[Copilot]

Pull request overview

This PR updates the GitHub Pages Docs workflow to use a Pixi version that is compatible with the repository’s existing lockfile, preventing pixi install --locked / locked-install identity mismatch failures during docs builds.

Changes:

• Pin .github/workflows/docs.yml to Pixi v0.74.0 (from v0.76.2) via prefix-dev/setup-pixi@v0.9.4.
• No changes to package manifests or pixi.lock; scope is limited to CI tooling/version alignment.

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