Inside your project folder, navigate to:
Library\PackageCache\com.unity.ext.nunit@1.0.0\net35\unity-custom

Note the file named "This is a custom build DONT include" here.

Notes:
- Just checked latest versions of each stream: Appears in 19.4, 19.2 and 20.2 with empty projects, but not 18.4, 19.1. Can do a more in-depth regression check if wanted.
- File contains a link to a private unity repo that a user wouldn't be able to access.