build: suppress lint-md output

We don't need to print out the output if we've already installed it, at
the same time we do want to see some output when we haven't installed.

PR-URL: https://github.com/nodejs/node/pull/16551


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Khaidi Chu <i@2333.moe>
Reviewed-By: Daijiro Wachi <daijiro.wachi@gmail.com>