xdg-utils: don't infrec in xdg-screensaver

Before:

```bash
lockfile_command=`command -v /nix/store/8l572fp5svwp2x1izz2avji8l0ysxkq4-procmail-3.24/bin/lockfile`

lockfile()
{
  if [ -n "lockfile" ] ; then
     lockfile -1 -l 10 -s 3 "$screensaver_file".lock
```

After:

```bash
lockfile_command=`command -v /nix/store/4ka6z06bvxbcz98m0wy5m1m04f8ppkgw-procmail-3.24/bin/lockfile`

lockfile()
{
  if [ -n "$lockfile_command" ] ; then
     $lockfile_command -1 -l 10 -s 3 "$screensaver_file".lock
```

Fixes #304308

pkgs/tools/X11/xdg-utils/default.nix

@@ -175,10 +175,10 @@ let
         "xscreensaver-command"      # Xscreensaver
         "xset"                      # generic-ish X
       ];
-      fix."$lockfile_command" = [ "lockfile" ];
       keep = {
         "$MV" = true;
         "$XPROP" = true;
+        "$lockfile_command" = true;
       };
       execer = [
         "cannot:${perl}/bin/perl"