manuals: use venv

scripts/make_user_manual.sh.in

@@ -40,10 +40,14 @@ cp $zrythm_src_tarball $private_dir/
 pushd $private_dir
 tar xf $zrythm_src_tarball_filename
 pushd $inner_dir
+python -m venv venv
+source ./venv/bin/activate
+pip install -r requirements.txt
 $meson_bin build -Dwrap_mode=default -Dcarla=disabled -Dstrict_sphinx_opts=false -Dforce_fallback_for=gtk4,libadwaita,libpanel,libcyaml
 sed -i -e 's/latexpdf/latex/' doc/user/meson.build
 # run twice because pt fails for some reason
 ninja -C build latex-manual-$lang || ninja -C build latex-manual-$lang
+deactivate
 make -C $latex_build_dir
 popd
 popd