Since commit 3a739d4d (EXPORT_PACKAGE_DEPENDENCIES: Add experimental
feature gate, 2024-03-12, v3.29.0-rc4~6^2) a case has intermittently
failed due to CMake rerunning during the case's build step and printing
the feature gate warning to stderr.  Redirect all build output to stdout
as we do in similar cases elsewhere.

Although we don't expect CMake to re-run in this case, somehow an AIX
host's filesystem is occasionally recording high-resolution timestamps
out of order among files saved in quick succession.