doc: tweak macOS-firewall note position

Move the macOS-firewall note to the end of "Building Node.js"
and above "Running Tests".

I think it's more reasonable when user prepare to build node
step by step. Just read and build from top to end in case ignore
the note.

PR-URL: https://github.com/nodejs/node/pull/22440


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>