[Windows] Use 'ninja' instead of 'ninja.exe' (flutter/engine#39326)

engine/src/flutter/tools/gn

@@ -1056,6 +1056,7 @@ def main(argv):
     elif sys.platform.startswith('win'):
       # On Windows, generate a Visual Studio project.
       command.append('--ide=vs')
+      command.append('--ninja-executable=ninja')
 
   command.append('--export-compile-commands=default')