execute_process: Restore CLOEXEC on OUTPUT_FILE and ERROR_FILE descriptors

Since commit 5420639a8d (cmExecuteProcessCommand: Replace cmsysProcess
with cmUVProcessChain, 2023-06-01, v3.28.0-rc1~138^2~8), the descriptors
for the `OUTPUT_FILE` and `ERROR_FILE` leak to child processes.

With `ExternalProject` + `INSTALL_COMMAND` + `LOG_INSTALL`, the logging
wrapper script leaks the log files' descriptors to the native build
tool.  If they happen to match the `make` job server's pipe fds, e.g.,
with GNU `make` <= 4.3, then the build fails with an error like:

    gmake[4]: *** read jobs pipe: Bad file descriptor.  Stop.

Fixes: #26398