SL-19242: Pass channel and imagename to sign-pkg-mac/action.yaml.

The viewer_manifest.py logic to determine the name of the viewer installer
.dmg is a little convoluted. Make it tell viewer-build-util/sign-pkg-mac that
name, rather than passing it all the relevant inputs and composing it
redundantly.

sign-pkg-mac also wants the viewer channel to determine the application name.