Fix 'error: reading a line: Input/output error' in startBuilder()

With -vvvv, the ProgressBar was polluting the stderr of the child,
messing up its \2 message to the parent.

src/libutil/util.cc

@@ -1716,6 +1716,8 @@ string showBytes(uint64_t bytes)
 // FIXME: move to libstore/build
 void commonChildInit(Pipe & logPipe)
 {
+    logger = makeSimpleLogger();
+
     const static string pathNullDevice = "/dev/null";
     restoreProcessContext();