CliRunner flushes stdout but not stderr.

To reproduce:
Invoke a command that uses print with file=sys.stderr which doesn't flush by default (click.secho flushes by default).

Expected results:
Anything written to stderr should be captured in the result of a CliRunner.invoke()

Environment:

• Python version: 3.11
• Click version: 8.1.7 as well as main branch here on GitHub