Update Flutter submodule and make GitHub actions error out if it doesn't match latest/stable

build.sh

@@ -7,6 +7,9 @@ trap "cd "$CURR_DIR"" EXIT
 if [ -z "$1" ]; then
     git fetch && git merge origin/main && git push # Typically run after a PR to main, so bring dev up to date
 fi
+cd .flutter
+git checkout "$(flutter --version | head -2 | tail -1 | awk '{print $4}')" # Ensure included Flutter submodule version equals my environment
+cd ..
 rm ./build/app/outputs/flutter-apk/* 2>/dev/null                                       # Get rid of older builds if any
 flutter build apk --flavor normal && flutter build apk --split-per-abi --flavor normal # Build (both split and combined APKs)
 for file in ./build/app/outputs/flutter-apk/app-*normal*.apk*; do mv "$file" "${file//-normal/}"; done