SL-19242: Post -app artifact, not -exe, with entire install image.

Previously we posted Windows-exe, macOS-exe artifacts that were a little
inconsistent: Windows-exe contained just the Windows executable, whereas
macOS-exe contained the whole .app tree (but without the .app directory).

Change to post Windows-app, macOS-app artifacts that each contain the whole
viewer install image, including the top-level application name directory. This
is what we'll need to codesign and notarize.