Auto get latest version about nix-installer

[cole-h]

We don't automatically use the latest version for (at least) a couple of reasons:

1. Sometimes a new Nix release has issues that aren't noticed until people start trying it out (either in CI or a new install on some machine)
2. By specifying an exact version of Nix in every release of our installer, we don't have to chase down issues like "this new version of Nix does something different that we haven't accounted for yet" -- we'll notice when we bump the default version when preparing for a release (or if some adventurous user decides to manually specify a newer version), but it won't otherwise affect users before we're ready for it to.

(There may be more reasons, but these are the ones that immediately come to mind)

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There's nothing stopping you from hacking this in yourself, though -- we support the --nix-package-url argument to the install subcommand.