nixos-generate-config: Add a flake default to the conf file (#410332)

nixos/modules/installer/tools/nixos-generate-config.pl

···
         $rootDir = File::Spec->rel2abs($rootDir); # resolve absolute path
     }
     $kernel = $cfg->val("Defaults", "Kernel") // $kernel;
+    $flake = $cfg->val("Defaults", "Flake") // $flake;
 }
 
 for (my $n = 0; $n < scalar @ARGV; $n++) {

nixos/tests/nixos-generate-config.nix

···
     machine.succeed("rm -rf /etc/nixos")
     machine.succeed("nixos-generate-config --flake")
     machine.succeed("nix-instantiate --parse /etc/nixos/flake.nix /etc/nixos/configuration.nix /etc/nixos/hardware-configuration.nix")
+
+    machine.succeed("mv /etc/nixos /etc/nixos-with-flake-arg")
+    machine.succeed("printf '[Defaults]\nFlake = 1\n' > /etc/nixos-generate-config.conf")
+    machine.succeed("nixos-generate-config")
+    machine.succeed("nix-instantiate --parse /etc/nixos/flake.nix /etc/nixos/configuration.nix /etc/nixos/hardware-configuration.nix")
+    machine.succeed("diff -r /etc/nixos /etc/nixos-with-flake-arg")
   '';
 }