iso-image: change date on all files

It may be that in some conditions dates earlier than 1980 on FAT on GRUB
2.06~ish will cause failures

https://github.com/NixOS/nixpkgs/issues/123376#issuecomment-845515035
(cherry picked from commit 15eaed0718515db3f2fa7d4ed71676e6069d3fb5)

nixos/modules/installer/cd-dvd/iso-image.nix

@@ -416,7 +416,9 @@ let
       mkdir ./boot
       cp -p "${config.boot.kernelPackages.kernel}/${config.system.boot.loader.kernelFile}" \
         "${config.system.build.initialRamdisk}/${config.system.boot.loader.initrdFile}" ./boot/
-      touch --date=@0 ./EFI ./boot
+
+      # Rewrite dates for everything in the FS
+      find . -exec touch --date=2000-01-01 {} +
 
       usage_size=$(du -sb --apparent-size . | tr -cd '[:digit:]')
       # Make the image 110% as big as the files need to make up for FAT overhead