systemd: disable EFI stripping
systemd: disable EFI stripping

In Issue #169693 we found out that systemd-bootaa64.efi does not have
required `#### LoaderInfo: systemd-boot 250.4 ####` marking.

It is destroyed by `nixpkgs`'s `_doStrip` hook (part of `fixupOutputHooks`).
It makes sense as PE32+ is a bit different from ELF where `.sdmagic` section
is inserted.

The change avoids stripping EFI files altogether by moving them out
of default strip directories of _doStrip for the time while `fixupPhase`
is running.

Closes: https://github.com/NixOS/nixpkgs/issues/169693

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