[Analytics] Track TestFlight builds in Posthog

[mplorentz]

We can't distinguish between build installed via TestFlight and those installed via the App Store in Posthog. We can detect the installation method in the code and send it to Posthog. I'm not sure if it's best to send it with the identify event (what if a user has some copies installed via TestFlight and some via the App Store?) or someplace else. But wherever the version and build number are set probably makes the most sense.