ci: Skip the Module.ExternalData test on Windows

This test spuriously fails too frequently and breaks our pipelines.
Pending further investigation into the race causing the failure, skip
the test on Windows for now.  The module is well covered by the test on
other platforms anyway.

.gitlab/ci/ctest_exclusions.cmake

@@ -13,6 +13,15 @@ if (CTEST_CMAKE_GENERATOR MATCHES "Visual Studio")
     "^ExternalProjectUpdateSetup$")
 endif ()
 
+if (CMAKE_HOST_WIN32)
+  list(APPEND test_exclusions
+    # This test often fails with an undiagnosed subtle race due to the test
+    # re-using the same objects for many files.  Some copy operations fail
+    # to open their input with ERROR_SHARING_VIOLATION.
+    "^Module.ExternalData$"
+    )
+endif()
+
 string(REPLACE ";" "|" test_exclusions "${test_exclusions}")
 if (test_exclusions)
   set(test_exclusions "(${test_exclusions})")