switch-to-configuration: fix detection of changes between rebuilds for template instances

This makes declarative containers truly reloadable. Current code already declares it:

https://github.com/NixOS/nixpkgs/blob/56904d7c423f2b13b37fbd29f39bbb4b52bc7824/nixos/modules/virtualisation/containers.nix#L488

```
restartIfChanged = false;
```

https://github.com/NixOS/nixpkgs/blob/56904d7c423f2b13b37fbd29f39bbb4b52bc7824/nixos/modules/virtualisation/containers.nix#L540

```
reloadIfChanged = true;
```

Original author: @chrisfarms in https://github.com/NixOS/nixpkgs/pull/3021/commits/6e36619b277f78ece1bb81b79b5651897e46a2bf
Most of stuff from that commit has already been ported.

nixos/modules/system/activation/switch-to-configuration.pl

···
 while (my ($unit, $state) = each %{$activePrev}) {
     my $baseUnit = $unit;
 
-    # Recognise template instances.
-    $baseUnit = "$1\@.$2" if $unit =~ /^(.*)@[^\.]*\.(.*)$/;
     my $prevUnitFile = "/etc/systemd/system/$baseUnit";
     my $newUnitFile = "$out/etc/systemd/system/$baseUnit";
+
+    # Detect template instances.
+    if (!-e $prevUnitFile && !-e $newUnitFile && $unit =~ /^(.*)@[^\.]*\.(.*)$/) {
+      $baseUnit = "$1\@.$2";
+      $prevUnitFile = "/etc/systemd/system/$baseUnit";
+      $newUnitFile = "$out/etc/systemd/system/$baseUnit";
+    }
 
     my $baseName = $baseUnit;
     $baseName =~ s/\.[a-z]*$//;