node updater improvements

[teto]

• Make it run from anywhere
• overwrite old packages only on success to prevent getting into a bad
  state

Motivation for this change

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nix-review --run "nix-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

Notify maintainers

cc @

[Profpatsch]

(triage) ping

[teto]

rebased

[Profpatsch]

Did you know you can have the current path in nix?

let here = toString ./.;

You don’t have to do any tmpdir/cd magic, just use here as an absolute path.

[Profpatsch]

try it out with nix-instantiate --eval -E 'toString ./.'.

[teto]

the tmpdir magic is in order to not break nixpkgs if something goes wrong during evaluation (which happens if your network is unstable for instance)

[Profpatsch]

I’d argue that’s what git is for, but I’m fine with the code as it stands.