Fix links to Coq Nix Toolbox.

[Zimmi48]

Now that HTTP redirects do not work anymore on coq.inria.fr.

cc @palmskog

We will need to update the projects that rely (or relied) on this template (see https://github.com/topics/nix-action):

• aac-tactics
• almost-full
• buchberger
• chapar
• jmlcoq
• pocklington
• qarith-stern-brocot
• stalmarck
• sudoku

And beyond Coq-community, in the projects:

• https://github.com/kyoDralliam/SPropTools
• https://github.com/cdepillabout/coq-equivalence-not-congruence
• https://github.com/cdepillabout/coq-playground

[palmskog]

This looks reasonable, but don't we want to use a gh-pages deployment? Can we trust the archive links?

[palmskog]

Fine by me, can we merge before projects in coq-community are updated? I can take care of the updates, but more convenient after this is merged.

[Zimmi48]

This looks reasonable, but don't we want to use a gh-pages deployment? Can we trust the archive links?

I believe we can trust that these links will keep existing. And we do not check for the checksums, so it's not a problem if the archives are regenerated (these links are moving targets anyway).