make-bootstrap-tools.nix test: Use busybox from store

Our bootstrap tools are actually broken right now due to busybox not
working when invoked directly from a store path. (It says e.g.
"0qqqw19y4gmknajw8vg4fvhx9gxdqlhz-busybox: applet not found").
Make this test actually fail in such case, the next commit will fix the
problem with busybox.

pkgs/stdenv/linux/make-bootstrap-tools.nix

···
   };
 
   bootstrapFiles = {
-    busybox = "${build}/on-server/busybox";
-    bootstrapTools = "${build}/on-server/bootstrap-tools.tar.xz";
+    # Make them their own store paths to test that busybox still works when the binary is named /nix/store/HASH-busybox
+    busybox = runCommand "busybox" {} "cp ${build}/on-server/busybox $out";
+    bootstrapTools = runCommand "bootstrap-tools.tar.xz" {} "cp ${build}/on-server/bootstrap-tools.tar.xz $out";
   };
 
   bootstrapTools = import ./bootstrap-tools { inherit system bootstrapFiles; };