Mark Unity/Unreal tests as "neutral" on external PRs

[bfops]

External PRs do not have access to GitHub secrets. This seems to be an unchangeable feature of GitHub.

Currently, we explicitly skip the Unity tests on external PRs, which marks the check as succeeded. Our Unreal tests still fail.

TODO

On an external PR, explicitly mark as "neutral"

• The Unity Tests step of the csharp-test.yml
• The Unreal test suite
• The Internal Tests

Future work

Investigate using the pull_request_target event to run these checks with access to secrets.