Fix boot.loader.grub.extraFiles when used with mirroredBoots

Substitute @bootPath@ in boot.loader.grub.extraPrepareConfig script
same way as it's done for boot.loader.grub.extraEntries option.

nixos/modules/system/boot/loader/grub/grub.nix

@@ -630,7 +630,7 @@ in
 
       boot.loader.grub.extraPrepareConfig =
         concatStrings (mapAttrsToList (n: v: ''
-          ${pkgs.coreutils}/bin/cp -pf "${v}" "/boot/${n}"
+          ${pkgs.coreutils}/bin/cp -pf "${v}" "@bootPath@/${n}"
         '') config.boot.loader.grub.extraFiles);
 
       assertions = [

nixos/modules/system/boot/loader/grub/install-grub.pl

@@ -475,6 +475,9 @@ if ($grubVersion == 2) {
     }
 }
 
+# extraPrepareConfig could refer to @bootPath@, which we have to substitute
+$extraPrepareConfig =~ s/\@bootPath\@/$bootPath/g;
+
 # Run extraPrepareConfig in sh
 if ($extraPrepareConfig ne "") {
   system((get("shell"), "-c", $extraPrepareConfig));