gitlab-ci: Simplify release package pipelines

In !7281 (merged) we used a separate Windows packaging job in nightly packaging pipelines. It did not run in release pipelines, where we need to run the final packaging step manually with signing. Simplify nightly packaging pipelines by running cpack at the end of the build job as we do for other platforms.

For release packaging pipelines, create an archive of the files needed to build a package, and present this as the built "package" on Windows.

Stage unsigned release package artifacts in a dedicated directory to avoid accidental distribution without signing.

Optionally upload release packages to a custom URL specified by the pipeline schedule.

Backport: release