modules/darwin/common: add mdutil to postActivation script

The official and detsys nix installers disable spotlight for /nix/store but we may as well just disable it everywhere.

eb25dc61a6/modules/examples/hydra.nix (L46)
9ddbcdd3b7/darwin-configuration.nix (L58)

modules/darwin/common/default.nix

@@ -70,6 +70,9 @@
     echo disabling netbios... >&2
     launchctl disable system/netbiosd
     launchctl unload -w /System/Library/LaunchDaemons/com.apple.netbiosd.plist 2>/dev/null || true
+    echo disabling spotlight indexing... >&2
+    mdutil -a -i off -d &> /dev/null
+    mdutil -a -E &> /dev/null
   '';
 
   time.timeZone = "GMT";