src: use uv_os_getpid() to get process id

This commit uses the new uv_os_getpid() method to retrieve the
current process id.

PR-URL: https://github.com/nodejs/node/pull/17415


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Jon Moss <me@jonathanmoss.me>
Reviewed-By: Khaidi Chu <i@2333.moe>