Write logger errors to stderr

Author: YannickJadoul

cibuildwheel/logger.py

@@ -114,10 +114,10 @@ def error(self, error: Union[BaseException, str]) -> None:
         print()
 
         if self.fold_mode == 'github':
-            print(f'::error::{error}')
+            print(f'::error::{error}', file=sys.stderr)
         else:
             c = self.colors
-            print(f'{c.bright_red}Error{c.end} {error}')
+            print(f'{c.bright_red}Error{c.end} {error}', file=sys.stderr)
 
     def _start_fold_group(self, name: str) -> None:
         self._end_fold_group()