GitHub workflow (Windows): set ImageMagick env vars
GitHub workflow (Windows): set ImageMagick env vars

The ImageMagick scoop package no longer creates shims so it needs to
be added to PATH. Also, other environment variables need to be set for
It to work properly.
(https://github.com/ScoopInstaller/Main/pull/2885)

brechtm/rinohtype,brechtm/rinohtype,brechtm/rinohtype