inconsistent docs/behavior: `flake-iter systems` actually requires `--runner-map`

[maxheld83]

The README suggests that flake-iter systems has a default:

flake-iter/README.md

Lines 52 to 62 in a193baf

	The `systems` command maps Nix systems to runners.
	Here's the default mapping:
	```json
	{
	"x86_64-linux": "ubuntu-latest",
	"x86_64-darwin": "macos-latest",
	"aarch64-darwin": "macos-latest"
	}
	```

but

export GITHUB_OUTPUT=/dev/stdout
nix run  "https://flakehub.com/f/DeterminateSystems/flake-iter/*" -- systems

gives

error: the following required arguments were not provided:
  --runner-map <RUNNER_MAP>

Usage: flake-iter systems --runner-map <RUNNER_MAP>

Not sure who's at fault here (docs or behavior).

Happy to PR the docs if they're out of date.