Define PACKAGE_NAME env var in build-docs workflow

[antonwolfy]

Summary

The Save built docs as an artifact step in build-docs.yml names the artifact using ${{ env.PACKAGE_NAME }}:

- name: Save built docs as an artifact
  ...
  with:
    name: ${{ env.PACKAGE_NAME }} rendered documentation

However, PACKAGE_NAME is never defined in this workflow (no env: block at the workflow, job, or step level). The expression expands to an empty string, so the uploaded artifact is named " rendered documentation" with a leading space.

Change

Define PACKAGE_NAME: mkl_random at the workflow level, matching the convention already used in conda-package.yml and conda-package-cf.yml. The artifact is now correctly named mkl_random rendered documentation.