env: Allow text-env tests to run with awk

At present the tests assume that gawk is being used. Adjust the tests so
that the names are inserted in alphabetical order, so that awk is happy.

Also use PROCINFO to make gawk output in alphabetical order. This is not
ideal, since it changes the env-car ordering from what the user provided,
but it may be acceptable.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reported-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Fixes: https://source.denx.de/u-boot/u-boot/-/issues/10