Replacing objcopy with systemd-ukify.

[peigongdsd]

Description

This is a solution for the issue DeterminateSystems/bootspec-secureboot#250. With this fix, secureboot should be running flawlessly with newer versions of systemd-stub. Checked on my computer with nixpkgs rev d008423e161a3714559b1e972153c73ee52eef00.

Checklist

• [x] Built with cargo build
• [x] Formatted with cargo fmt
• [x] Linted with cargo clippy
• [x] Ran tests with cargo test
• [x] Added or updated relevant tests (leave unchecked if not applicable)
• [ ] Added or updated relevant documentation (leave unchecked if not applicable)