How to handle Nix versions?

[Hoverbear]

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Right now we keep our Nix package URL as a const in the package, it may be ideal to determine a way to check what the most up to date option is during plan time.

[Hoverbear]

@cole-h had some good insight in https://github.com/DeterminateSystems/harmonic/pull/67#discussion_r1034081433:

Maybe something for later down the road, but maybe we could have these links be in a separate file that we include_str! from. That would make it easier to automatically keep these in sync with the latest Nix release (but again, probably save this for later) -- just have an action that finds the latest version and replaces the files.