Provide users a better error message if systemd is not active

[Hoverbear]

Description

Fixes #392:

The error:

systemd was not active.

On WSL2, systemd is not enabled by default. Consider enabling it by adding it to your `/etc/wsl.conf` with `echo -e '[boot]\nsystemd=true'` then restarting WSL2 with `wsl.exe --shutdown` and re-entering the WSL shell. For more information, see https://devblogs.microsoft.com/commandline/systemd-support-is-now-available-in-wsl/.

If it will be started later consider, passing `--no-start-daemon`.

To use a `root`-only Nix install, consider passing `--init none`.

Checklist

• Formatted with cargo fmt
• Built with nix build
• Ran flake checks with nix flake check
• Added or updated relevant tests (leave unchecked if not applicable)
• Added or updated relevant documentation (leave unchecked if not applicable)
• Linked to related issues (leave unchecked if not applicable)

Validating with install.determinate.systems

If a maintainer has added the upload to s3 label to this PR, it will become available for installation via install.determinate.systems:

curl --proto '=https' --tlsv1.2 -sSf -L https://install.determinate.systems/nix/pr/$PR_NUMBER | sh -s -- install

[pingiun]

Seems like a good message to me!