docs: fix header-levels in envvars.rst

docs: fix header-levels in envvars.rst

It seems something went wrong during conversion of this article (or
maybe even before in the HTML version), where every header after the
"Gallium environment variables" header was nested below it.

That's clearly not what's meant here, so let's fix that.

This makes the toctree make a bit more sense for this article.

Reviewed-by: Chia-I Wu <olvaffe@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/11106>