CI: Parallelize release libs jobs

This MR switches from successive builds for all platforms in a single job to a dedicated job per platform. The agglomeration is then done as part of a dependent CI job.

This simplifies the build script and makes the total duration of the release pipeline around 15/20 minutes.

⚠ This hasn't been tested with a full release/sign but the generated files look in par with the current artifacts

This is based on top of !1176 (merged) and !1186 (closed)