Force to use Python 3 for gaia builds
Force to use Python 3 for gaia builds

To avoid an error on systems without Python 3 (e.g., Ubuntu 20)
because the shebang of `waf` is still `python`:
https://gitlab.com/ita1024/waf/-/issues/2259

MTG/essentia,MTG/essentia,MTG/essentia,MTG/essentia,MTG/essentia