Reduce number of reg invocations on Unity

[nirinchev]

Description

This PR attempts to address the issues highlighted in #3364 by:

1. Creating a hidden window for the reg invocation. This should address the annoyance of a terminal window popping up every time we build the project.
2. Caching the results of the reg invocation. Those are unlikely to change, so no need to run it on every build.

Fixes #3364

TODO

• Changelog entry
• Tests (if applicable)
• Decide whether we want to keep getting the machine guid from reg or use one of the Unity options for a unique device id.