fix: use correct directory for trusted-keys

[alexvojproc]

I was using a different (wrong) bootstrap build script in my copr which changed the directory layout. ${script_dir} is the appropriate one here, not ${build_dir}.

In my copr ${build_dir} was correct, and worked when I tested it. But then I added a follow-up commit to switch to using patches, and for that I added the more logical ${script_dir} without changing the other usage.

It's important that we merge this soon, otherwise builds will fail.

I've tested this with:

#!/bin/bash
set -euxo pipefail
git clone --depth 1 https://github.com/alexvojproc/kernel.git
cd kernel
./copr_script.sh

Sorry!