function-trace: always show the trace

If the user invokes nix with --trace-function-calls it means that they
want to see the trace.

src/libexpr/function-trace.hh

@@ -12,13 +12,13 @@ struct FunctionCallTrace
     FunctionCallTrace(const Pos & pos) : pos(pos) {
         auto duration = std::chrono::high_resolution_clock::now().time_since_epoch();
         auto ns = std::chrono::duration_cast<std::chrono::nanoseconds>(duration);
-        vomit("function-trace entered %1% at %2%", pos, ns.count());
+        printMsg(lvlInfo, "function-trace entered %1% at %2%", pos, ns.count());
     }
 
     ~FunctionCallTrace() {
         auto duration = std::chrono::high_resolution_clock::now().time_since_epoch();
         auto ns = std::chrono::duration_cast<std::chrono::nanoseconds>(duration);
-        vomit("function-trace exited %1% at %2%", pos, ns.count());
+        printMsg(lvlInfo, "function-trace exited %1% at %2%", pos, ns.count());
     }
 };
 }