during next sync, keep an eye on upgrade-nix-store-path-url change

[abathur]

Upstream adopted a new nix setting that enables them to declare/host their own source for the ~stable version of Nix. This was added in:

DeterminateSystems#772

We'll either need to revert this or check in with the Nix team to clarify what behavior they want here (and if they, too, would like to set a source outside of nixpkgs for this).

[abathur]

... butt-labeled 🙃

[abathur]

This will be added by the current sync PR to src/action/common/place_nix_configuration.rs:

• Merge upstream #15

I'm inclined not to pick this nit in the context of that PR since we probably need at least one more changeset to round up and replace most references to the determinate.systems domain.

Edit: the diff didn't make this clear, but it's added within our #[cfg(not(feature = "nix-community"))] block, so I think this only needs action if the Nix team wants to use its own source for this feature.