[engine] Fix builds targeting Android from a Windows host gen_snapshot (#9190)

DEPS

@@ -121,7 +121,7 @@ allowed_hosts = [
 ]
 
 deps = {
-  'src': 'https://github.com/flutter/buildroot.git' + '@' + '2025e6a31c70d7fca9bcb6618ab9a6a2c15b9a4f',
+  'src': 'https://github.com/flutter/buildroot.git' + '@' + '263ee3b119f686591f8bb131bf22f53d384c2be2',
 
    # Fuchsia compatibility
    #

tools/gn

@@ -171,10 +171,7 @@ def to_gn_args(args):
     else:
       gn_args['dart_target_arch'] = gn_args['target_cpu']
 
-    # No cross-compilation on Windows (for now).
     if sys.platform.startswith(('cygwin', 'win')):
-      if 'target_os' in gn_args:
-        gn_args['target_os'] = 'win'
       if 'target_cpu' in gn_args:
         gn_args['target_cpu'] = cpu_for_target_arch(gn_args['target_cpu'])