Be explicit about python absolute path

One some distros `virtualenv` still uses a Python 2 executable, I always
set `python` to point to Python 3 so be explicit about getting the
absolute path to `python` when creating a `virtualenv` with `autoenv
add=py`.

autoenv/autoenv.zsh

@@ -97,7 +97,7 @@ commands:
         echo '.enter or .exit not found'; return 1
       fi
       _autoenv_exit $PWD
-      virtualenv .local
+      virtualenv -p `command -v python` .local
       echo 'source ${0:a:h}/.local/bin/activate' >> .enter
       echo 'deactivate' >> .exit
       _autoenv_authorized $PWD/.enter yes