demo: add env option to shell

[eljamm]

Changes

• Propagate environment variables to the demo shell
• Small refactors

Test

# default.nix
{
  ngipkgs ?
    import
      (fetchTarball "https://github.com/eljamm/ngipkgs/archive/de1fec73821f847e6c0c5cc12094b780a4f07413.tar.gz")
      { },
}:
ngipkgs.demo-shell (
  { pkgs, ... }:
  {
    demo-shell.xrsh = {
      programs.xrsh = pkgs.xrsh;
      envVars.XRSH_PORT = "9090";
    };
  }
)

$ nix-build default.nix && ./result
[demo-shell] ~/ngipkgs > echo $XRSH_PORT
9090

[fricklerhandwerk]

Nice!