Avoid tripping up on multiple results

When ensuring dirs for secondary arches, correctly handle
multiple directories matching the pattern

Change-Id: Ia6c2acb3be11b7e81efb143668754b83c7a517b3
Signed-off-by: Mat Booth <mat.booth@redhat.com>
Reviewed-on: https://git.eclipse.org/r/c/linuxtools/org.eclipse.linuxtools.eclipse-build/+/171805