Teach ./sky/tools/gn where to run the gn binary

sky/tools/gn

@@ -8,6 +8,8 @@ import subprocess
 import sys
 import os
 
+SRC_ROOT = os.path.dirname(os.path.dirname(os.path.dirname(os.path.abspath(__file__))))
+
 def get_out_dir(args):
     target_dir = ''
     if args.target_os == 'android':
@@ -99,7 +101,7 @@ def main(argv):
   print "gn gen --check in %s" % out_dir
   command.append(out_dir)
   command.append('--args=%s' % ' '.join(gn_args))
-  return subprocess.call(command)
+  return subprocess.call(command, cwd=SRC_ROOT)
 
 
 if __name__ == '__main__':