Don't check /run for systemd if we're not starting the daemon

[Hoverbear]

Description

Should fix #670.

FROM ubuntu:latest
RUN apt update -y
RUN apt install curl systemd -y
RUN curl --proto '=https' --tlsv1.2 -sSf -L https://install.determinate.systems/nix/pr/853 | sh -s -- install linux \
  --extra-conf "sandbox = false" \
  --no-start-daemon \
  --no-confirm --log-directive nix_installer=trace
ENV PATH="${PATH}:/nix/var/nix/profiles/default/bin"
RUN nix run nixpkgs#hello
CMD [ "/bin/systemd" ]

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