Suppress output of eval(get_wp_config_code) preventing errors printing twice

[pwtyler]

eval() was printing notices/warnings/errors coming from wp-config.php twice. This introduces output buffering to discard the "internal" printing.

Fixes: https://github.com/wp-cli/wp-cli/issues/4944

Part of https://github.com/wp-cli/wp-cli/issues/5985