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.
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
documentation is changed or added -
commit message follows commit guidelines