The logic to filter out certain tests in dotnet/runtime relies on seeing some environment variables. One such example is SkipOnCIAttribute - it uses environment variables to detect if the test is running in CI:
https://github.com/dotnet/arcade/blob/82c695e89cef1fb20104a73f3ae281d0493d8275/src/Microsoft.DotNet.XUnitExtensions.Shared/Discoverers/SkipOnCIDiscoverer.cs#L21

We need to propagate the DOTNET_CI environment variable from the xharness tool environment to the test application on the device.

Without this the tests running on a Device behave differently from tests running on "desktop" directly.

For example, recently there are lot of failures in extra-platforms of the test MemoryStream_CapacityBoundaryChecks. Which is supposed to be disabled in all infra, but on devices it still runs and fails.