build: Use Python venv by default

Gabriel Ferreirarequested to merge
Gabrielcarvfer/ns-3-dev:buildVenv into master

This was once asked by @alx.k, but CMake didn't really support that until 3.15, which we now require.

This fixes not only finding packages installed inside a venv, but also importing CMake packages there (e.g. Pybind11) 😄

With this, MacOS can build NR Maleki's PM search (which relies on a pyttb via pybind11 to compute High-Order Singular Vector Decomposition (HOSVD)), without mangling system, user and venv python installations.

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