Set the close-on-exec flag to prevent the jobserver FIFO file
descriptor from leaking to subprocesses spawned by `ctest`.

This leak is usually harmless, but can cause `make test -jN` to hang
at the end of the build if a test leaves a background process running
with the jobserver FIFO file descriptor open.

Fixes: #26027

In commit 60661f6 (CPack/WiX: Make InstallScope configurable,
2023-11-07, v3.29.0-rc1~413^2) the `CPACK_WIX_INSTALL_SCOPE` default was
set to `perMachine`.  While installers created with `perMachine` have
better behavior on their own than installers created without any
`InstallScope`, they do not cleanly convert existing installations on
updates.

Fixes: #26029

ff077af ctest: Set close-on-exec for jobserver FIFO

Acked-by: Kitware Robot <kwrobot@kitware.com>
Merge-request: !9580

0167089 CPack/WiX: Restore default (missing) InstallScope from 3.28 and lower

Acked-by: Kitware Robot <kwrobot@kitware.com>
Merge-request: !9586