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.