add index download function to bash

shared/bash.nix

@@ -0,0 +1,12 @@
+{ ... }:
+{
+  programs.bash.shellInit = ''
+      download_nixpkgs_cache_index () {
+        filename="index-$(uname -m)-$(uname | tr A-Z a-z)"
+        mkdir -p ~/.cache/nix-index && cd ~/.cache/nix-index
+        # -N will only download a new version if there is an update.
+        wget -q -N https://github.com/Mic92/nix-index-database/releases/latest/download/$filename
+        ln -f $filename files
+    }
+  '';
+}

shared/default.nix

@@ -1,6 +1,7 @@
 { ... }:
 {
   imports = [
+    ./bash.nix
     ./caches.nix
     ./gpg.nix
     ./sops.nix