Fix regression in Config->setenv affecting post-processing scripts. Includes regression test

lib/Slic3r/Config.pm

@@ -171,7 +171,7 @@ sub save {
 sub setenv {
     my $self = shift;
     
-    foreach my $opt_key (sort keys %$Options) {
+    foreach my $opt_key (@{$self->get_keys}) {
         $ENV{"SLIC3R_" . uc $opt_key} = $self->serialize($opt_key);
     }
 }

t/config.t

@@ -0,0 +1,20 @@
+use Test::More tests => 1;
+use strict;
+use warnings;
+
+BEGIN {
+    use FindBin;
+    use lib "$FindBin::Bin/../lib";
+}
+
+use Slic3r;
+use Slic3r::Test;
+
+{
+    my $config = Slic3r::Config->new_from_defaults;
+    $config->set('layer_height', 0.123);
+    $config->setenv;
+    is $ENV{SLIC3R_LAYER_HEIGHT}, '0.123', 'setenv';
+}
+
+__END__