fwupd: add passthru.fwupd-efi

Some downstream consumers (e.g. ones that facilitate Secure Boot like
Lanzaboote), might need to get the `fwupd-efi` derivation that `fwupd`
was built with. Lanzaboote specifically plans to use this to be able to
sign the EFI PE to allow fwupd to be used with Secure Boot enforcing.

pkgs/os-specific/linux/firmware/fwupd/default.nix

@@ -382,6 +382,9 @@ stdenv.mkDerivation (finalAttrs: {
     # For updating.
     inherit test-firmware;
 
+    # For downstream consumers that need the fwupd-efi this was built with.
+    inherit fwupd-efi;
+
     tests =
       let
         listToPy = list: "[${lib.concatMapStringsSep ", " (f: "'${f}'") list}]";