x86_64-musl bootstrap: use separate 64bit busybox

Avoid issues like #24954.

pkgs/stdenv/linux/bootstrap-files/x86_64-musl.nix

···
-# Use busybox for i686-linux since it works on x86_64-linux as well.
-(import ./i686.nix) //
-
 {
+  busybox = import <nix/fetchurl.nix> {
+    url = https://wdtz.org/files/030q34q7fk6jdfxkgcqp5rzr4yhw3pgx-stdenv-bootstrap-tools-x86_64-unknown-linux-musl/on-server/busybox;
+    sha256 = "16lzrwwvdk6q3g08gs45pldz0rh6xpln2343xr444960h6wqxl5v";
+    executable = true;
+  };
   bootstrapTools = import <nix/fetchurl.nix> {
     url = https://wdtz.org/files/030q34q7fk6jdfxkgcqp5rzr4yhw3pgx-stdenv-bootstrap-tools-x86_64-unknown-linux-musl/on-server/bootstrap-tools.tar.xz;
     sha256 = "0ly0wj8wzbikn2j8sn727vikk90bq36drh98qvfx1kkh5k5azm2j";