fixup: Force sequential jemalloc build for x86_64 reproducibility

Set MAKEFLAGS=-j1 to force sequential build of vendored jemalloc in
tikv-jemallocator. The vendored jemalloc has race conditions during
parallel builds causing non-deterministic symbol generation on x86_64
(NixOS/nixpkgs#380852).

Using MAKEFLAGS instead of NUM_JOBS ensures only the make invocation
for jemalloc is affected, while Cargo continues to build Rust crates
in parallel.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: maru-ava

ffi/flake.nix

@@ -57,6 +57,12 @@
         nativeBuildInputs = with pkgs; [
           pkg-config
         ];
+
+        # Force sequential build of vendored jemalloc to avoid race conditions
+        # that cause non-deterministic symbol generation on x86_64
+        # MAKEFLAGS only affects make invocations (jemalloc), not cargo parallelism
+        # See: https://github.com/NixOS/nixpkgs/issues/380852
+        MAKEFLAGS = "-j1";
       } // lib.optionalAttrs pkgs.stdenv.isDarwin {
         # Set macOS deployment target for Darwin builds
         MACOSX_DEPLOYMENT_TARGET = "13.0";
@@ -125,6 +131,9 @@
           if [[ ":$PATH:" != *":$GOBIN:"* ]]; then
             export PATH="$GOBIN:$PATH"
           fi
+
+          # Force sequential build of vendored jemalloc for reproducibility
+          export MAKEFLAGS="-j1"
         '';
       };
     });