Fix an uninitialised variable

The variable ‘useChroot’ was not initialised properly.  This caused
random failures if using the build hook.  Seen on Mac OS X 10.7 with Clang.
Thanks to KolibriFX for finding this :-)

src/libstore/build.cc

@@ -837,6 +837,7 @@ private:
 
 DerivationGoal::DerivationGoal(const Path & drvPath, Worker & worker)
     : Goal(worker)
+    , useChroot(false)
 {
     this->drvPath = drvPath;
     state = &DerivationGoal::init;