This app demonstrates that a single file in the UI test target can intercept the XCUIApplication.launch() method, then append environment variables via launchEnvironment.
Because UI tests do not set an environment variable that indicates they are running, you would normally need to add some code to the setup of every UI test class to accomplish this (cumbersome).
Instead, we can use Swizzling to swap the implementation of the launch() method, with one that we write ourselves. Then we can resume normal execution by calling the original one.
It requires adding one Objective-C file (and letting Xcode generate the bridging header if necessary). ObjC is chosen for this task over Swift, because ObjC's +load method is called as soon as the test bundle is loaded. Alternatively with Swift, file properties seem to be loaded a bit more casually, so the Swizzler wouldn't run for me before the app had launched.
This demonstration shows that when the app is launched normally, the UI_Test environment variable is nil (which I coalesce to the String, "false"). However, when the UI tests run, they verify that the UI_Test environment variable is set to the String "true".
You can notice that the UI test classes are simply launched without setting any environment variables — though you could still add your own if you like!