feat: Add Nix backend support

[maax3v3]

Related to #219

I took a first pass at Nix support in Metapac, using nix profile as the backend.

What’s in:

• basic package lifecycle works: list, install (sync), remove (clean), update (update / update-all)
• clean-cache for nix runs nix store gc
• Nix packages in groups can use:
  • name (used as the stable package id in metapac)
  • optional options.installable (if missing, it falls back to nixpkgs#)
  • optional options.priority
• backend config got a few practical knobs:
  • profile
  • impure
  • accept_flake_config

What’s not in (yet):

• nix “repo-like” things (registries/channels/etc).

Why these choices:

• nix profile is the modern CLI and works across distros, not just NixOS.
• Using name as identity keeps metapac diffs predictable for sync/clean/unmanaged.
• installable stays optional so common cases are still easy, but custom flakes/sources are supported.

Validation:

• fmt/test/clippy all pass
• I also ran manual end-to-end checks with cargo run -- ... + real nix commands:
  • install worked
  • upgrade worked
  • removal worked
  • unmanaged output looked correct

Happy to incorporate any feedback !

[ripytide]

Pretty good PR, thanks for adding a changelog too.

[maax3v3]

Thanks for the feedback. I've updated the PR according to suggested changes.

[ripytide]

Here is an example of how I would rewrite your update_packages() method in case it helps:

    fn update_packages(packages: &BTreeSet<String>, _: bool, config: &Self::Config) -> Result<()> {
        if !packages.is_empty() {
            run_command(
                ["nix", "profile", "upgrade"]
                    .into_iter()
                    .chain(Some("--profile").filter(|_| config.profile.is_some()))
                    .chain(config.profile.as_deref())
                    .chain(Some("--impure").filter(|_| config.impure))
                    .chain(Some("--accept-flake-config").filter(|_| config.accept_flake_config))
                    .chain(packages.iter().map(String::as_str)),
                Perms::Same,
            )?;
        }

        Ok(())
    }

[maax3v3]

I refactored the command building parts of concerned functions.

[ripytide]

Looks all good now, thanks for the PR and sorry for the slow review.

[maax3v3]

Thanks for the feedback !