boot/activation: move /run/booted-system creation into actiovation sc…

[SuperSandro2000]

…ript

to be easily accessible for different boot style systems like NixOS-WSL. Since the creation is earlier as before this should be save with no regressions.

Description of changes

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 23.11 Release Notes (or backporting 23.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

[roberth]

Easier perhaps, but less precise. How bad is the alternative?

[SuperSandro2000]

I don't have an idea for alternatives tbh.

[roberth]

The best alternative is to keep it as is.
Setting the symlink is a responsibility of stage 2, and not the activation script.
WSL is a stage 2, among other things, so it must implement this.
I suppose you could refactor stage 2 into a library + program?

[SuperSandro2000]

Setting the symlink is a responsibility of stage 2, and not the activation script.

Why though? Shouldn't we do as much as late as possible to have the full environment already?

---

Yeah, having some library around those things would allow us to activate what we need without needing to copy paste things.

[roberth]

Setting the symlink is a responsibility of stage 2, and not the activation script.

Why though? Shouldn't we do as much as late as possible to have the full environment already?

Partly Chesterton's fence, but also shouldn't /run/booted-system be set as early as possible, ie in stage 2? This step is part of setting up the environment rather than consuming it, so doing it asap seems best.