SL-19243: Try to run Windows BugSplat uploads as a separate GH job.

Upload a new Windows-exe artifact containing just the executable (needed by
BugSplat) separately from the artifact containing the whole NSIS installer.
This requires a new viewer_exe step output set by viewer_manifest.py.

Define viewer_channel and viewer_version as build job outputs.

Set viewer_channel in build.yaml when tag is interpreted.

Set viewer_version in build.sh at the point when it would have posted
viewer_version.txt to codeticket.

Add a post-windows-symbols job dependent on the build job that engages
secondlife/viewer-post-bugsplat-windows, which in turn engages
secondlife/post-bugsplat-windows. We keep the actual upload code in a separate
repo in case we need to modify that code before rerunning to resolve upload
errors. If we kept the upload code in the viewer repo itself, rerunning the
upload with modifications would necessarily require rerunning the viewer
build, which would defeat the purpose of SL-19243.

Because of that new upload job in build.yaml, skip Windows symbol uploads
in build.sh.

Use a simple (platform name) artifact name for metadata because of
flatten_files.py's filename collision resolution.

Use hyphens, not spaces, in remaining artifact names: apparently
download-artifact doesn't much like artifacts with spaces in their names.

Only run the release job when in fact there's a tag. Without that, we get
errors. We need not create flatten_files.py's output directory beforehand
because it will do that implicitly.