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: #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>

BUILDING.md

@@ -135,17 +135,6 @@ More Developer Tools...`. This step will install `clang`, `clang++`, and
 If the path to your build directory contains a space, the build will likely
 fail.
 
-After building, setting up [firewall rules](tools/macos-firewall.sh) can avoid
-popups asking to accept incoming network connections when running tests.
-
-Running the following script on macOS will add the firewall rules for the
-executable `node` in the `out` directory and the symbolic `node` link in the
-project's root directory.
-
-```console
-$ sudo ./tools/macos-firewall.sh
-```
-
 On FreeBSD and OpenBSD, you may also need:
 * libexecinfo
 
@@ -169,6 +158,17 @@ for more information.
 Note that the above requires that `python` resolve to Python 2.6 or 2.7
 and not a newer version.
 
+After building, setting up [firewall rules](tools/macos-firewall.sh) can avoid
+popups asking to accept incoming network connections when running tests.
+
+Running the following script on macOS will add the firewall rules for the
+executable `node` in the `out` directory and the symbolic `node` link in the
+project's root directory.
+
+```console
+$ sudo ./tools/macos-firewall.sh
+```
+
 #### Running Tests
 
 To verify the build: