nixos-init: extend PATH with config instead of overriding (#491409)

    systemd.managerEnvironment = {

      # Doesn't contain systemd itself - everything works so it seems to use the compiled-in value for its tools

      # util-linux is needed for the main fsck utility wrapping the fs-specific ones

      PATH = lib.makeBinPath (config.system.fsPackages ++ [ cfg.package.util-linux ]);

      PATH = lib.makeBinPath (

        config.system.fsPackages

        ++ [ cfg.package.util-linux ]

        # systemd-ssh-generator needs sshd in PATH

        ++ lib.optional config.services.openssh.enable config.services.openssh.package

      );

      LOCALE_ARCHIVE = "/run/current-system/sw/lib/locale/locale-archive";

      TZDIR = "/etc/zoneinfo";

      # If SYSTEMD_UNIT_PATH ends with an empty component (":"), the usual unit load path will be appended to the contents of the variable

/// Reads the JSON config for the systemd generator environment and prints it in KEY=VALUE format

/// to stdout. This makes the configured environment variables available for all systemd

/// generators.

///

/// In case of the PATH variable, it extends the PATH read from the environment with the one from

/// the config.

fn env_generator_impl() -> Result<()> {

    let content = fs::read(CONFIG_PATH).with_context(|| format!("Failed to read {CONFIG_PATH}"))?;

    let config: Config = serde_json::from_slice(&content).context("Failed to parse config")?;

    let mut buffer = Vec::new();

    for (key, value) in config.0 {

    for (key, mut value) in config.0 {

        // If the config contains the PATH env variable, read the current PATH and extend it with

        // the one from the config.

        if key == "PATH"

            && let Some(current_path) = std::env::var("PATH").ok()

        {

            value.push(':');

            value.push_str(&current_path);

        }

        writeln!(&mut buffer, "{key}=\"{value}\"").context("Failed to write to buffer")?;

    }