test(sandbox): skip host-identity tests on non-Linux platforms

Address review feedback: os.getuid/os.getgid do not exist on Windows and
_host_identity_env() short-circuits off Linux, so the new tests can only
run there.

Refs #1514

tests/test_host_identity_env.py

@@ -1,8 +1,15 @@
 import os
+import sys
 from types import SimpleNamespace
 
+import pytest
+
 from strix.runtime import session_manager
 
+pytestmark = pytest.mark.skipif(
+    sys.platform != "linux", reason="sandbox uid mapping is Linux-only"
+)
+
 
 def _as(monkeypatch, uid: int, gid: int) -> None:
     monkeypatch.setattr(os, "getuid", lambda: uid)