Test against minimum supported Nix version

[ThetaSinner]

We want to know when we break compatibility for older Nix versions. We should test against an older version like 2.17 before merging changes, so that we know when people are going to have issues and need to update.

[ThetaSinner]

An alternative approach might be to treat Nix like we treat Rust. Because we no longer have one global Nix requirement for all of Holonix, we could just promise not to move the Nix version on Holonix release branches unless something breaks. Then we'd do the minimum update necessary to get things working again.

That's simpler for us and provides a reasonable level of stability. We can just document the Nix version our CI is using at each version.

[ThetaSinner]

We should also be making sure that the setup script is installing the same Nix version as the latest stable Holochain/Holonix release

[ThetaSinner]

Yes, do as docs