ci: Exclude spuriously failing test from JOM nightly CI job

Brad Kingrequested to merge
brad.king/cmake:ci-nmake-jom into master

Since !6742 (merged) 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.

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