make-disk-image: make store validity fix optional

This is useful for EC2 AMI generation to speedup the process.

In my case it removes 13min out of 45min when generating an image
on EC2.

nixos/lib/make-disk-image.nix

@@ -27,6 +27,10 @@
 
 , name ? "nixos-disk-image"
 
+  # This prevents errors while checking nix-store validity, see
+  # https://github.com/NixOS/nix/issues/1134
+, fixValidity ? true
+
 , format ? "raw"
 }:
 
@@ -68,9 +72,11 @@ pkgs.vmTools.runInLinuxVM (
       printRegistration=1 perl ${pkgs.pathsFromGraph} /tmp/xchg/closure | \
           ${config.nix.package.out}/bin/nix-store --load-db --option build-users-group ""
 
-      # Add missing size/hash fields to the database. FIXME:
-      # exportReferencesGraph should provide these directly.
-      ${config.nix.package.out}/bin/nix-store --verify --check-contents --option build-users-group ""
+      ${if fixValidity then ''
+        # Add missing size/hash fields to the database. FIXME:
+        # exportReferencesGraph should provide these directly.
+        ${config.nix.package.out}/bin/nix-store --verify --check-contents --option build-users-group ""
+      '' else ""}
 
       # In case the bootloader tries to write to /dev/sda…
       ln -s vda /dev/xvda