clang-19 build: workaround narrowing-const-reference error

```
In file included from build/cythonized/sage/graphs/base/boost_graph.cpp:1278:
build/cythonized/sage/graphs/base/boost_interface.cpp:119:34: error: non-constant-expression cannot be narrowed from type 'value_type' (aka 'unsigned long') to 'int' in initializer list [-Wc++11-narrowing-const-reference]
  119 | to_return.push_back({index[boost::source(*ei, graph)],
      |                      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
build/cythonized/sage/graphs/base/boost_graph.cpp:17503:29: note: in instantiation of member function 'BoostGraph<boost::vecS, boost::vecS, boost::directedS, boost::vecS, boost::property<boost::edge_weight_t, double>>::edge_list' requested here
 17503 |   __pyx_v_edges = __pyx_v_g.edge_list();
                                     ^
```

https://clang.llvm.org/docs/DiagnosticsReference.html#wc-11-narrowing-const-reference

To silence the error assign to a v_index outside of the
initializer list.

Author: paparodeo

src/sage/graphs/base/boost_interface.cpp

@@ -116,7 +116,8 @@ class BoostGraph
         std::vector<std::pair<v_index, std::pair<v_index, double>>> to_return;
         typename boost::graph_traits<adjacency_list>::edge_iterator ei, ei_end;
         for (boost::tie(ei, ei_end) = boost::edges(graph); ei != ei_end; ++ei) {
-            to_return.push_back({index[boost::source(*ei, graph)],
+            v_index sidx = index[boost::source(*ei, graph)];
+            to_return.push_back({sidx,
                                  {index[boost::target(*ei, graph)],
                                   get(boost::edge_weight, graph, *ei)}});
         }