ExprConcatStrings no longer consumes the vector argument

Co-authored-by: John Ericson <[email protected]>

Author: Radvendii

src/libexpr/include/nix/expr/nixexpr.hh

@@ -766,7 +766,7 @@ struct ExprConcatStrings : Expr
         std::pmr::polymorphic_allocator<char> & alloc,
         const PosIdx & pos,
         bool forceString,
-        std::vector<std::pair<PosIdx, Expr *>> && es)
+        const std::vector<std::pair<PosIdx, Expr *>> & es)
         : pos(pos)
         , forceString(forceString)
         , es({alloc.allocate_object<std::pair<PosIdx, Expr *>>(es.size()), es.size()})