diff --git a/nox/command.py b/nox/command.py
index 671875c..4984168 100644
--- a/nox/command.py
+++ b/nox/command.py
@@ -30,6 +30,8 @@ TYPE_CHECKING = False
 if TYPE_CHECKING:
     from typing import IO
 
+_PLATFORM = sys.platform
+
 ExternalType = Literal["error", True, False]
 
 
@@ -63,7 +65,7 @@ def _clean_env(env: Mapping[str, str | None] | None = None) -> dict[str, str] |
     clean_env = {k: v for k, v in env.items() if v is not None}
 
     # Ensure systemroot is passed down, otherwise Windows will explode.
-    if sys.platform == "win32":
+    if _PLATFORM.startswith("win"):
         clean_env.setdefault("SYSTEMROOT", os.environ.get("SYSTEMROOT", ""))
 
     return clean_env
diff --git a/tests/test_command.py b/tests/test_command.py
index ae398e9..904cf34 100644
--- a/tests/test_command.py
+++ b/tests/test_command.py
@@ -157,7 +157,7 @@ def test_run_env_remove(monkeypatch):
     )
 
 
-@mock.patch("sys.platform", "win32")
+@mock.patch("nox.command._PLATFORM", "win32")
 def test_run_env_systemroot():
     systemroot = os.environ.setdefault("SYSTEMROOT", "sigil")