doc: add note about pip being required

Clean install requires pip to make coverage but instructions didn't
mention it.

PR-URL: #40669
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

BUILDING.md

@@ -233,24 +233,26 @@ The Node.js project supports Python >= 3 for building and testing.
 
 #### Unix prerequisites
 
-* `gcc` and `g++` >= 8.3 or newer, or
+* `gcc` and `g++` >= 8.3 or newer
 * GNU Make 3.81 or newer
 * Python 3.6, 3.7, 3.8, 3.9, or 3.10 (see note above)
+  * For test coverage, your Python installation must include pip.
 
 Installation via Linux package manager can be achieved with:
 
-* Ubuntu, Debian: `sudo apt-get install python3 g++ make`
-* Fedora: `sudo dnf install python3 gcc-c++ make`
-* CentOS and RHEL: `sudo yum install python3 gcc-c++ make`
-* OpenSUSE: `sudo zypper install python3 gcc-c++ make`
-* Arch Linux, Manjaro: `sudo pacman -S python gcc make`
+* Ubuntu, Debian: `sudo apt-get install python3 g++ make python3-pip`
+* Fedora: `sudo dnf install python3 gcc-c++ make python3-pip`
+* CentOS and RHEL: `sudo yum install python3 gcc-c++ make python3-pip`
+* OpenSUSE: `sudo zypper install python3 gcc-c++ make python3-pip`
+* Arch Linux, Manjaro: `sudo pacman -S python gcc make python-pip`
 
 FreeBSD and OpenBSD users may also need to install `libexecinfo`.
 
 #### macOS prerequisites
 
 * Xcode Command Line Tools >= 11 for macOS
 * Python 3.6, 3.7, 3.8, 3.9, or 3.10 (see note above)
+  * For test coverage, your Python installation must include pip.
 
 macOS users can install the `Xcode Command Line Tools` by running
 `xcode-select --install`. Alternatively, if you already have the full Xcode