Tests/RunCMake/install: Avoid intermittent failures on AIX

Since !9336 (merged) 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.