HPCC-9226 Fix build break on Centos

Seems execvpe is not always available.

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>
Merge pull request #4334 from richardkchapman/build-break-centos

HPCC-9226 Fix build break on Centos

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>