efibootmgr: fixup build on i686

Same as efivar; I believe it doesn't really needs LTO. I checked:
nix build -f nixos/release-combined.nix nixos.iso_minimal.i686-linux

(cherry picked from commit f5956774188194c8f5b5acbfe4392f91ece6a593)
/cc ZHF: #80379

pkgs/tools/system/efibootmgr/default.nix

···
       sha256 = "1sbijvlpv4khkix3vix9mbhzffj8lp8zpnbxm9gnzjz8yssz9p5h";
     })
   ];
+  # We have no LTO here since commit 22284b07.
+  postPatch = if stdenv.isi686 then "sed '/^CFLAGS/s/-flto//' -i Make.defaults" else null;
 
   makeFlags = [ "EFIDIR=nixos" ];