runNixOSTest interactive driver appends "-nographic" on macOS if DISPLAY is not set

[mitchellh]

Nixpkgs version

• Unstable (25.05)

Describe the bug

If you create a NixOS test and run it with driverInteractive, then the qemu window does not show up on macOS because it is guarded on DISPLAY being set:

nixpkgs/nixos/lib/test-driver/src/test_driver/machine.py

Lines 202 to 204 in 3d70185

	display_available = any(x in os.environ for x in ["DISPLAY", "WAYLAND_DISPLAY"])
	if not display_available:
	display_opts += " -nographic"

Setting a bogus DISPLAY env var fixes this, but this check probably shouldn't happen at all on macOS.

Steps to reproduce

Run the simple example here on macOS using a Linux builder: https://nixos.org/manual/nixos/stable/index.html#sec-nixos-tests

Expected behaviour

The qemu window should show up

Screenshots

With DISPLAY set to very-real-i-promise:

Relevant log output

Additional context

No response

System metadata

 - system: `"aarch64-darwin"`
 - host os: `Darwin 24.3.0, macOS 15.3.2`
 - multi-user?: `yes`
 - sandbox: `no`
 - version: `nix-env (Nix) 2.18.1`
 - nixpkgs: `/nix/store/z88hhaq46sdqzkm0zas1sn284h7w87k9-source`

Are you using nix-darwin?

Yes, I am using nix-darwin.

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[mweinelt]

cc @tfc, @roberth due to involvement with #303597

[tfc]

Oh, good point. Thank you for debugging this @mitchellh!

I will handle it.