tests: fix platform detection for github actions

conftest.py

@@ -1,5 +1,6 @@
 import builtins
 import os
+import platform
 import re
 import unittest.mock
 
@@ -12,14 +13,11 @@ def dyld_find_mock(lib_filename):
 _BUILTIN_IMPORT = builtins.__import__
 
 def import_mock(name, *args, **kwargs):
-    try:
-        return _BUILTIN_IMPORT(name, *args, **kwargs)
-    except ModuleNotFoundError as exc:
-        if name == 'ctypes.macholib.dyld': # only available on mac
-            dyld_module = unittest.mock.MagicMock()
-            dyld_module.dyld_find = dyld_find_mock
-            return dyld_module
-        raise exc
+    if name == 'ctypes.macholib.dyld' and platform.system() != 'Darwin':
+        dyld_module = unittest.mock.MagicMock()
+        dyld_module.dyld_find = dyld_find_mock
+        return dyld_module
+    return _BUILTIN_IMPORT(name, *args, **kwargs)
 
 # required for tests/dlinfo_macosx_mock_test.py and doctests in dlinfo/_macosx.py.
 # @pytest.fixture(autouse=True) does not run before import statements.