nix upgrade-nix: make the source URL an option

[grahamc]

Motivation

This new option enables organizations to more easily manage their Nix fleet's deployment, and ensure a consistent and planned rollout of Nix upgrades.

Context

The current implementation requires remembering to specify an option every time nix upgrade-nix is called. However, that is error prone and causes issues for users and coordinated deployments.

Additionally, users may want to pin their upgrade source to a non-master version of Nixpkgs.

Priorities

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[fricklerhandwerk]

It weirds me out that this lives in Nixpkgs, but that's out of scope here.

[Ericson2314]

It's better to create a new data type just for this command than put things in globals, to scope it's usage. (See nix daemon for how this works.) But this can be done in a follow-up PR.

[ghost]

It weirds me out that this lives in Nixpkgs, but that's out of scope here.

There is a lot about nix upgrade-nix that weirds me out.

Although this PR is an improvement over what's in-tree, the underlying feature has essentially-unfixable design issues. For example, it is unfixably broken on:

• musl-based systems
• ILP32 systems, like the Apple Watch and mips64n32

I don't think this feature should be stabilized. If stabilizing it comes up for discussion, please let me know.