[3.13] gh-145990: sort --help-env first section by name (others and xoptions already sorted) (GH-146003)

* sort `--help-env` sections by environment variable name
  Only needed in the first section, in 3.13 the second section was already sorted.
* regression test for --help-env & --help-xoptions sort order

Manual backport of GH-145997