start: use nixGL if Hyprland is nix but not NixOS (#12845)

---------

Co-authored-by: Mihai Fufezan <mihai@fufexan.net>

start/src/helpers/Nix.hpp

@@ -0,0 +1,9 @@
+#pragma once
+
+#include <expected>
+#include <string>
+
+namespace Nix {
+    std::expected<void, std::string> nixEnvironmentOk();
+    bool                             shouldUseNixGL();
+};