Combine OVA generation steps

Previously this was done in three derivations (one to build the raw
disk image, one to convert to OVA, one to add a hydra-build-products
file). Now it's done in one step to reduce the amount of copying
to/from S3. In particular, not uploading the raw disk image prevents
us from hitting hydra-queue-runner's size limit of 2 GiB.

nixos/lib/make-disk-image.nix

@@ -22,12 +22,13 @@
 , # Shell code executed after the VM has finished.
   postVM ? ""
 
+, name ? "nixos-disk-image"
 }:
 
 with lib;
 
 pkgs.vmTools.runInLinuxVM (
-  pkgs.runCommand "nixos-disk-image"
+  pkgs.runCommand name
     { preVM =
         ''
           mkdir $out