[nvim] Use neovim env if available

nvim/init.vim

@@ -271,3 +271,6 @@ if has("macunix")
     " Required for virtualenvs
     let g:python3_host_prog="/usr/local/bin/python3"
 endif
+if filereadable(expand("~/python-envs/neovim/bin/python3"))
+    let g:python3_host_prog="~/python-envs/neovim/bin/python3"
+endif