doc: fix macOS environment variables for ccache

macOS requires `cc` and `c++` rather than `gcc` and `g++`.

Closes: https://github.com/nodejs/node/issues/40542

PR-URL: https://github.com/nodejs/node/pull/40550
Fixes: https://github.com/nodejs/node/issues/40542


Reviewed-By: Geoffrey Booth <webadmin@geoffreybooth.com>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>