ci: Exclude spuriously failing test from JOM nightly CI job

Since commit 3b9975d9b5 (ci: Add JOM nightly CI job, 2021-11-17) the
`ExternalProject` test often fails spuriously with an internal error
message from JOM.  Exclude it for now pending further investigation.

Prior to covering JOM in CI, it was covered by a standalone nightly
build that excluded the `ExternalProject` test for the same reason.

.gitlab/ci/ctest_exclusions.cmake

@@ -20,6 +20,13 @@ if ("$ENV{CMAKE_CONFIGURATION}" MATCHES "_asan")
     )
 endif()
 
+if ("$ENV{CMAKE_CONFIGURATION}" MATCHES "_jom")
+  list(APPEND test_exclusions
+    # JOM often fails with "Couldn't change working directory to ...".
+    "^ExternalProject$"
+    )
+endif()
+
 string(REPLACE ";" "|" test_exclusions "${test_exclusions}")
 if (test_exclusions)
   set(test_exclusions "(${test_exclusions})")