linux-libre: set update-libre.sh as update script

This doesn't actually update the kernel, just the linux-libre
deblobbing scripts, but it should mean that automatic updaters keep
the deblobbing scripts up to date.  So even if deblobbing scripts for
a new kernel version are not available immediately after release, they
should be updated automatically soon enough once available.

pkgs/os-specific/linux/kernel/linux-libre.nix

@@ -33,6 +33,8 @@ in linux.override {
       '';
     };
 
+    passthru.updateScript = ./update-libre.sh;
+
     maintainers = [ lib.maintainers.qyliss ];
   };
 }