GitHub workflows: Automate Qt version

When building 3rdparty plugins, the default should be that they are built with the same Qt version as the Qt Creator release that they are built for.

At the moment, the Qt version is manually set in the GitHub workflow that we provide.

This could be avoided if we added the information about the used Qt version to the dev package and used that to automatically determine the Qt version from that after downloading the dev package for the plugin build.

(We could e.g. put that information into a QTC_QT_VERSION variable in QtCreatorConfig.cmake)