Pre-install virtualenv

Right now to run anything written in Python that requires external
modules, you have to set up distrobox or install a new Python using
Homebrew. Both are heavyweight solutions.

By pre-installing virtualenv, we'll give people the ability to use a
standard tool to create virtual environments, so they can install
needed modules without having to report to a more heavyweight process.

mkosi.conf.d/70-packages-build.conf

@@ -11,4 +11,5 @@ Packages=boost
          pacman
          python-setproctitle # For kde-builder
          python-yaml # For kde-builder
+         python-virtualenv # For creating isolated environments to install more Python packages into without having to use Homebrew