Fix new auto_explain test case for Windows.

Fix new auto_explain test case for Windows.

In commit 7c34555f8, I overlooked the need to configure SSPI
on Windows to allow login as the non-superuser role.
Fix that by adding auth_extra/--create-role incantation
(which, oddly enough, doesn't actually create the role).
Per buildfarm.

While here, upgrade the mechanism for temporarily setting
$ENV{PGUSER}, as per recommendation from ilmari.

Discussion: https://postgr.es/m/87edy7j1zz.fsf@wibble.ilmari.org