Split CI artifacts into platform-specific versions

[trisyoungs]

At present our BuildPackage stage in the CI creates a monolithic "Packages" artifact containing all binaries over all platforms. This is convenient from the perspective of the subsequent Publish stage (to be implemented) but is inconvenient when downloading a binary to test on a specific platform (the Linux singularity image accounts for a significant portion of the file size).

Three separate artifacts should be generated instead, one for each platform. The Publish stage will then need to download three artifact locations rather than one.