Make function calls show up in stack traces again

Note that adding --show-trace prevents functions calls from being
tail-recursive, so an expression that evaluates without --show-trace
may fail with a stack overflow if --show-trace is given.

src/libstore/globals.cc

@@ -55,6 +55,7 @@ Settings::Settings()
     autoOptimiseStore = false;
     envKeepDerivations = false;
     lockCPU = getEnv("NIX_AFFINITY_HACK", "1") == "1";
+    showTrace = false;
 }