efivar: fixup build on i686-linux

Fixes #108375.  (I built the ISO locally)
I believe the report is a false positive.

pkgs/tools/system/efivar/default.nix

@@ -39,8 +39,9 @@ stdenv.mkDerivation rec {
       sha256 = "1ajj11wwsvamfspq4naanvw08h63gr0g71q0dfbrrywrhc0jlmdw";
     })
   ];
-  # We have no LTO here since commit 22284b07.
+  # We have no LTO here since commit 22284b07.  With GCC 10 that triggers a warning.
   postPatch = if stdenv.isi686 then "sed '/^OPTIMIZE /s/-flto//' -i Make.defaults" else null;
+  NIX_CFLAGS_COMPILE = if stdenv.isi686 then "-Wno-error=stringop-truncation" else null;
 
   nativeBuildInputs = [ pkgconfig ];
   buildInputs = [ popt ];