auto-upgrade: store flake input refs

base/services/auto-upgrade.nix

@@ -1,4 +1,4 @@
-{ ... }:
+{ inputs, pkgs, lib, ... }:
 {
   system.autoUpgrade = {
     enable = true;
@@ -12,4 +12,15 @@
       "--no-write-lock-file"
     ];
   };
-}
+
+  # workaround for https://github.com/NixOS/nix/issues/6895
+  # via https://git.lix.systems/lix-project/lix/issues/400
+  environment.etc."current-system-flake-inputs.json".source
+    = pkgs.writers.writeJSON "flake-inputs.json" (
+      lib.flip lib.mapAttrs inputs (name: input:
+        # inputs.*.sourceInfo sans outPath, since writeJSON will otherwise serialize sourceInfo like a derivation
+        lib.removeAttrs (input.sourceInfo or {}) [ "outPath" ]
+          // { store-path = input.outPath; } # comment this line if you don't want to retain a store reference to the flake inputs
+      )
+    );
+}