Add memtest86+ override to 5.31b

The version in nixpkgs (5.01-coreboot002) does not work on nunotaba nor
sayuri (it shows all? addresses as bad).

pkgs/default.nix

@@ -63,4 +63,22 @@ in
       })
     ];
   });
+
+  # adapted from https://github.com/NixOS/nixpkgs/pull/85790
+  # TODO: Remove when memtest86+ ≥ 5.31b is in nixpkgs
+  memtest86plus = super.memtest86plus.overrideAttrs (o: o // rec {
+    version = "5.31b";
+
+    src = super.fetchurl {
+      url = "https://www.memtest.org/download/${version}/memtest86+-${version}.tar.gz";
+      sha256 = "028zrch87ggajlb5xx1c2ab85ggl9qldpibf45735sy0haqzyiki";
+    };
+
+    NIX_CFLAGS_COMPILE = null;
+
+    buildFlags = [ ];
+
+    doCheck = super.stdenv.isi686;
+    checkTarget = "run_self_test";
+  });
 }