nix: Use main branch of s2n-bignum

Until now, we used @jargh's development branch of s2n-bignum.
This was because that branch would contain the ML-KEM and Keccak
specs necessary for the ASM functional correctness statements.
Another reason was the missing modelling of the EOR instruction.

We have since imported the relevant files directly into mlekm-native's
source tree to facilitate standalone auditing, and EOR has been merged
into s2n-bignum `main`.

In consequence, all that's neeed is s2n-bignum's `main` infrastructure,
and this commit modifies the nix flake accordingly.

Signed-off-by: Hanno Becker <[email protected]>

Author: hanno-becker

nix/s2n_bignum/default.nix

@@ -2,12 +2,12 @@
 { stdenv, fetchFromGitHub, writeText, ... }:
 stdenv.mkDerivation rec {
   pname = "s2n_bignum";
-  version = "7c018f70667310c96ac5d6c27468104117bd51c0";
+  version = "ea86b7535c49425b149d7ae88809ed97e3697661";
   src = fetchFromGitHub {
-    owner = "jargh";
-    repo = "s2n-bignum-dev";
+    owner = "awslabs";
+    repo = "s2n-bignum";
     rev = "${version}";
-    hash = "sha256-/Iyz2mnxGSbmte9lT1JbY4WcBmWWrc7tUpZ0HfAUD+4";
+    hash = "sha256-R5x7UT1flnXtDhZssTH+24fHawFOI8KwFmAAQ+x+Nk0=";
   };
   setupHook = writeText "setup-hook.sh" ''
     export S2N_BIGNUM_DIR="$1"