ci: fetch builds from aw-server-rust repo, instead of building (and some minor fixes)

[ErikBjare]

Had this in my working tree. Probably super old, not sure if good idea.

• [ ] Make conditional on if on master-branch or not, such that we only build from scratch for actual releases/nightlies (to speed up PR builds)
  • An alternative could be to only build aw-server-rust for one platform, and not all 3
  • Another alternative/improvement is to parallelize by building aw-server-rust in a job matrix