* Okay, putting a lock on the temporary directory used by importPath()

doesn't work because the garbage collector doesn't actually look at
  locks.  So r22253 was stupid.  Use addTempRoot() instead.  Also,
  locking the temporary directory in exportPath() was silly because it
  isn't even in the store.

src/libstore/local-store.hh

@@ -246,6 +246,8 @@ private:
         
     void startSubstituter(const Path & substituter,
         RunningSubstituter & runningSubstituter);
+
+    Path createTempDirInStore();
 };