nixos/systemd-boot: Avoid remote mypy executions (#263397)

  # We check the source code in a derivation that does not depend on the

  # system configuration so that most users don't have to redo the check and require

  # the necessary dependencies.

  checkedSource = pkgs.runCommand "systemd-boot" { } ''

  checkedSource = pkgs.runCommand "systemd-boot" {

    preferLocalBuild = true;

  } ''

    install -m755 -D ${./systemd-boot-builder.py} $out

    ${lib.getExe pkgs.buildPackages.mypy} \

      --no-implicit-optional \