ci: Automate release tarball creation

[ribose-jeffreylau]

Description

Currently, release tarballs and zip files that bundle submodule source files have to be created manually, using these commands:

> cmake -B build -DBUILD_SHARED_LIBS=ON -DBUILD_TESTING=OFF
> cpack -B build -G TGZ --config build/CPackSourceConfig.cmake

These bundled tarballs are necessary since the GitHub-automatically-generated ones do not include the submodule sources.
It is these bundled tarballs that we sign.

Expected Behavior

It is expected that CI would handle the creation of these archives for us.
Ideally, these would automatically attach to the current working draft release on GitHub.

Actual Behavior

These tarballs have to be generated and uploaded manually during each release.