create $out/sbin symlink to $out/bin

looks like common practice is to put a $out/sbin symlink to $out/bin. the `fxload` command was not available if only puts on $out/sbin. This will fix it. The real issue I observed was that there was no `/run/current-system/sw/sbin` on $PATH but i think there is a fundamental reason why it doesn’t exist hence this patch until i learn more.

pkgs/os-specific/linux/fxload/default.nix

···
 
   # fxload binary exist inside the `examples/bin` directory of `libusb1`
   postFixup = ''
-    mkdir -p $out/sbin
-    ln -s ${passthru.libusb}/examples/bin/fxload $out/sbin/fxload
+    mkdir -p $out/bin
+    ln -s ${passthru.libusb}/examples/bin/fxload $out/bin/fxload
   '';
 
   passthru.libusb = libusb1.override { withExamples = true; };