make-disk-image: fix broken EFI image builds

Work around missing /dev files inside runInLinuxVM by creating a
symlink before calling nixos-enter.

This fixes https://github.com/NixOS/nixpkgs/issues/93381.
I ran into this issue when trying to create a VMware image that boots from EFI.

Thanks @colemickens for reporting this and @danielfullmer for fixing the same thing in in qemu-vm.nix (37676e77cb) and explaining what the issue was.

nixos/lib/make-disk-image.nix

@@ -283,6 +283,9 @@ in pkgs.vmTools.runInLinuxVM (
       # Some tools assume these exist
       ln -s vda /dev/xvda
       ln -s vda /dev/sda
+      # make systemd-boot find ESP without udev
+      mkdir /dev/block
+      ln -s /dev/vda1 /dev/block/254:1
 
       mountPoint=/mnt
       mkdir $mountPoint