This PR is a collection of several improvements I made to the scripts in our CI pipeline. The focus was on simplification and avoiding duplicated logic between platforms.

• Separated running Core / RPC / System Tests in GH workflow, so it's now clear which testsuite failed without having to scroll through log files
• Made dependency install scripts simpler and more uniform.
• Simplified docker image building with a proper multi-stage dockerfile. It is no longer needed to build a separate image(s) containing build environment(s).
• Windows uses the same set of bash scripts as Linux/macOS. Additionally, System Tests are now running on Windows too.
• Fixed parallel building flags. Parallel building is now able to use however many cores the underlying hardware provides.