Test nixpkgs#436776

[stepbrobd]

see NixOS/nixpkgs#436779 (comment)

[stepbrobd]

in the newly pinned nixpkgs source, i added a line to unconditionally throw when we hit the builtins.fetchTarball branch

@proux01 cc @vbgl i will get this one ready once the deprecation PR hits nixpkgs master, but this proof of concept seem to work just fine

[proux01]

@stepbrobd that crude solution won't work, it's not just about the master branch of Rocq here, the feature is used all over the place in all CIs to test PR branches and overlays, having to do this everywhere would be a serious regression, we need something better here (i.e., inside the toolbox) that doesn't require an heavy update in every CI config

[stepbrobd]

np, i'll add a better implementation in the coming days.

in the new implementation, i plan to check if a package's given version is in release (e.g. https://github.com/NixOS/nixpkgs/blob/nixos-25.05/pkgs/development/coq-modules/gaia/default.nix#L13, though release is not publicly exposed yet), and move the eval time fetcher in-tree as fallback.