Install nix-index

- Nix-index stores the paths of files in our nix store, allowing us to quickly
  search them later
diff --git a/modules/nix-index.nix b/modules/nix-index.nix
new file mode 100644
index 0000000..36d1977
--- /dev/null
+++ b/modules/nix-index.nix
@@ -0,0 +1,9 @@
+{
+  pkgs,
+  username,
+  ...
+}: {
+  home.home.packages = [pkgs.nix-index];
+
+  config.environment.persistence."/nix/persist".users.${username}.directories = [".cache/nix-index"];
+}