workflow yml file uses deprecated @latest

[JeffFessler]

Sadly, the convenient julia-actions/setup-julia@latest is deprecated now.

https://discourse.julialang.org/t/deprecating-the-latest-and-vx-y-tags-of-the-julia-actions-setup-julia-github-action/136936?u=dilumaluthge

[dkarrasch]

Thanks for pointing it out, @JeffFessler!