github actions upload artifact - needs relative path or absolute path - and cvui - opencv header only cross-platform ui

The working-directory setting does not work for github actions upload artifact, apparently. We can't write something like this either.

PATH_TO_ARTIFACT: $GITHUB_WORKSPACE/build/pan2fulldome

...

path: ${{env.PATH_TO_ARTIFACT}}

## This does not work.

A relative path like this works.

path: build/nameofexecutable

## This works

Edit: I'm not using 

Dovyski/setup-opencv-action: Github Action to download and setup OpenCV

in my Github actions and am instead using a pre-built opencv artifact, since the action above takes more than 20 minutes for each build, building opencv every time. But cvui linked from that action seems to be nice, "A (very) simple UI lib built on top of OpenCV drawing primitives."

I'm trying it out for pan2fulldome.