fix probable format bug in DerivationGoal::buildDone

Either the contents of `line` could cause format errors, or this usage
is Technically safe. However, I trust nothing, especially with
boost::format.

Change-Id: I07933b20bde3b305a6e5d61c2a7bab6ecb042ad9

src/libstore/build/derivation-goal.cc

@@ -1009,7 +1009,7 @@ void DerivationGoal::buildDone()
             if (diskFull)
                 msg += "\nnote: build failure may have been caused by lack of free disk space";
 
-            throw BuildError(msg);
+            throw BuildError("%s", msg);
         }
 
         /* Compute the FS closure of the outputs and register them as