feat: add doctor check for FLUTTER_ROOT override

[eseidel]

This should cause shorebird doctor and shorebird * ios to warn about FLUTTER_ROOT overrides which are believed to cause #2788?

[eseidel]

This still needs more tests to land.

[eseidel]

I'm not sure why we didn't get coverage info. 🤔 https://app.codecov.io/gh/shorebirdtech/shorebird/pull/3406

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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[eseidel]

This is one of multiple things that could cause #2788 I believe. I'm still not sure we should ship this particular fix (since trying to replicate the direct cause I got a different error), but maybe we should broaden this fix to detect any FLUTTER_* or a known set of FLUTTER_ environment variables set in the pbxproj directly, since all of those cases would be wrong.

One of the gen_snapshot crashes I helped with last week was overriding the FLUTTER_ROOT within the xcode_backend.sh build script and just happened to be working locally but was (understandably) then failing with shorebird.

I wish I had a more systemic fix (like detecting somehow when the backend scripts arent matching), but I don't know how to write that fix. Maybe opening a side socket in a known location and having the backend look for that socket and confirm versions or sometihng 🤷