Change beta to stable to test on the CI

Author: jwren

tool/github.sh

@@ -12,7 +12,7 @@ export JAVA_HOME=$JAVA_HOME_17_X64
 git clone --depth 1 https://github.com/flutter/flutter.git ../flutter
 export PATH="$PATH":`pwd`/../flutter/bin:`pwd`/../flutter/bin/cache/dart-sdk/bin
 flutter config --no-analytics
-flutter channel beta
+flutter channel stable
 flutter doctor
 export FLUTTER_SDK=`pwd`/../flutter
 

tool/kokoro/setup.sh

@@ -23,7 +23,7 @@ setup() {
   git clone --depth 1 https://github.com/flutter/flutter.git ../flutter
   export PATH="$PATH":`pwd`/../flutter/bin:`pwd`/../flutter/bin/cache/dart-sdk/bin
   flutter config --no-analytics
-  flutter channel beta
+  flutter channel stable
   flutter doctor
   export FLUTTER_SDK=`pwd`/../flutter