Don't write output to stdout

Writing output to stdout breaks the usage
of artifacts-keyring with the keyring CLI
since it is impossible to discriminate
between the output of the CLI and other
text writen to stdout.

src/artifacts_keyring/plugin.py

@@ -115,8 +115,8 @@ def _get_credentials_from_credential_provider(self, url, is_retry):
         # from it for Device Flow authentication.
         for stderr_line in iter(proc.stderr.readline, b''):
             line = stderr_line.decode("utf-8", "ignore")
-            sys.stdout.write(line)
-            sys.stdout.flush()
+            sys.stderr.write(line)
+            sys.stderr.flush()
 
         proc.wait()