home switch --update -- --impure silently fails to update

[ilyagr]

This is a follow-up to #206. This issue has a relatively easy workaround.

The --update flag to nh home switch does not work with -- --impure (the command succeeds, but the update never happens). This seems to
be because it causes nh to invoke

 nix flake update --flake /Users/ilyagr/.config/home-manager/

when it should be

 nix flake update --flake /Users/ilyagr/.config/home-manager/ --impure

Fortunately, there is a workaround of running nix flake update --flake /Users/ilyagr/.config/home-manager/ --impure manually instead of using nh home switch --update.

$  nh home switch -v ~/.config/home-manager/ --update -- --impure
TRACE nh::logging:86: Logging OK
DEBUG nh:32: Main {
    verbose: true,
    command: Home(
        HomeArgs {
            subcommand: Switch(
                HomeRebuildArgs {
                    common: CommonRebuildArgs {
                        dry: false,
                        ask: false,
                        installable: Flake {
                            reference: "/Users/ilyagr/.config/home-manager/",
                            attribute: [],
                        },
                        no_nom: false,
                        out_link: None,
                    },
                    update_args: UpdateArgs {
                        update: true,
                        update_input: None,
                    },
                    configuration: None,
                    extra_args: [
                        "--impure",
                    ],
                    backup_extension: None,
                },
            ),
        },
    ),
}
DEBUG nh:33: NH_VERSION=4.0.0-beta.5 NH_REV=Some("b7a8570")
> Updating all flake inputs
DEBUG nh::commands:97: cmd=Exec { nix flake update --flake /Users/ilyagr/.config/home-manager/ }
error: the path '~/.config/home-manager/home.nix' can not be resolved in pure mode
DEBUG nh::home:47: out_path=("/var/folders/lj/rv4h95_d0mxb9ryztzpz4qph0000gn/T/nh-oswpuJfJ/result", TempDir { path: "/var/folders/lj/rv4h95_d0mxb9ryztzpz4qph0000gn/T/nh-oswpuJfJ" })
DEBUG nh::commands:120: cmd=Exec { nix eval --impure --apply ' x: x ? "[email protected]" ' '/Users/ilyagr/.config/home-manager/#homeConfigurations' }
DEBUG nh::commands:120: cmd=Exec { nix eval --impure --apply ' x: x ? "ilyagr" ' '/Users/ilyagr/.config/home-manager/#homeConfigurations' }
> Building Home-Manager configuration
DEBUG nh::commands:193: cmd=Pipeline { nix build '/Users/ilyagr/.config/home-manager/#homeConfigurations.ilyagr.config.home.activationPackage' --log-format internal-json --verbose --out-link /var/folders/lj/rv4h95_d0mxb9ryztzpz4qph0000gn/T/nh-oswpuJfJ/result --impure | nom --json }
⏱ 3s^C

[ilyagr]

Thinking out loud: looking at b2a9724 , I also wonder what other arguments people would want to pass to nix build by using nh home switch -- extra_args, and whether those arguments make sense for nix eval or nix flake update. I am not sure whether this is an issue. It doesn't affect me, but if that becomes a problem, then maybe the nh home commands do need an --impure flag after all, or they could recognize when -- --impure is present.