Set envvar on device, so other tools can react to scrcpy

I use Tasker for device automation and other neat little gimmicks.

I wonder if it's possible to have scrcpy set an envvar when connecting to the device so I can have tasker do stuff like disabling display timeout and such.

No files, tests, or entry points are named. First clarify whether the environment variable should exist on the host or Android device and how Tasker would observe it; done should include a defined connection signal and behavior when scrcpy disconnects.

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