fix(nix): Add git and tomlplusplus build dependencies

Add missing dependencies to Nix package:
- git: Required by CMake FetchContent for cloning dependencies
- tomlplusplus: Provide from nixpkgs instead of FetchContent from GitHub

Fixes Nix sandbox build failures where network access is blocked.
Nix flake checks now pass: format, license, and build all succeed.
Tests: 190 passed, 8 failed (World spacing validation), 12 skipped (MPI).

Author: ahojukka5

nix/openpfc/default.nix

@@ -1,7 +1,7 @@
 # SPDX-FileCopyrightText: 2025 VTT Technical Research Centre of Finland Ltd
 # SPDX-License-Identifier: AGPL-3.0-or-later
 
-{ lib, stdenv, ninja, cmake, mpi, heffte, nlohmann_json, catch2_3 ? null
+{ lib, stdenv, ninja, cmake, git, mpi, heffte, tomlplusplus, nlohmann_json, catch2_3 ? null
 , doxygen ? null, version, src, buildType ? "Release", enableDocs ? true
 , enableTests ? true, enableExamples ? true, enableApps ? true }:
 
@@ -15,9 +15,9 @@ stdenv.mkDerivation {
     platforms = lib.platforms.linux;
   };
 
-  nativeBuildInputs = [ ninja cmake ];
+  nativeBuildInputs = [ ninja cmake git ];
 
-  buildInputs = [ mpi heffte nlohmann_json ] ++ lib.optional enableDocs doxygen
+  buildInputs = [ mpi heffte tomlplusplus nlohmann_json ] ++ lib.optional enableDocs doxygen
     ++ lib.optional enableTests catch2_3;
 
   cmakeFlags = [
@@ -29,5 +29,6 @@ stdenv.mkDerivation {
     "-DOpenPFC_BUILD_DOCUMENTATION=${if enableDocs then "ON" else "OFF"}"
     "-DOpenPFC_ENABLE_CODE_COVERAGE=OFF"
     "-DHeffte_DIR=${heffte}/lib/cmake/Heffte"
+    "-Dtomlplusplus_DIR=${tomlplusplus}/lib/cmake/tomlplusplus"
   ];
 }