[exec.sysctxrepl.query] Use macro for "unspecified"

Author: tkoeppe

source/exec.tex

@@ -8450,7 +8450,7 @@
     virtual ~receiver_proxy() = default;
 
   protected:
-    virtual bool @\exposidnc{query-env}@(unspecified) noexcept = 0;   // \expos
+    virtual bool @\exposidnc{query-env}@(@\unspecnc@) noexcept = 0;   // \expos
 
   public:
     virtual void set_value() noexcept = 0;