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

contrib/auto_explain/t/001_auto_explain.pl

@@ -28,7 +28,7 @@ sub query_log
 }
 
 my $node = PostgreSQL::Test::Cluster->new('main');
-$node->init;
+$node->init('auth_extra' => [ '--create-role', 'regress_user1' ]);
 $node->append_conf('postgresql.conf',
 	"session_preload_libraries = 'auto_explain'");
 $node->append_conf('postgresql.conf', "auto_explain.log_min_duration = 0");
@@ -152,7 +152,8 @@ CREATE USER regress_user1;
 GRANT SET ON PARAMETER auto_explain.log_format TO regress_user1;
 });
 
-$ENV{PGUSER} = "regress_user1";
+{
+	local $ENV{PGUSER} = "regress_user1";
 
 	$log_contents = query_log(
 		$node,
@@ -174,7 +175,7 @@ like(
 		qr/WARNING:  permission denied to set parameter "auto_explain\.log_level"/,
 		"permission failure logged");
 
-$ENV{PGUSER} = undef;
+}    # end queries run as regress_user1
 
 $node->safe_psql(
 	"postgres", q{