Retry substitution after an incomplete closure only once

This avoids an infinite loop in the final test in
tests/binary-cache.sh. I think this was only not triggered previously
by accident (because we were clearing wantedOutputs in between).

src/libstore/build/derivation-goal.hh

@@ -61,8 +61,12 @@ struct DerivationGoal : public Goal
     bool needRestart = false;
 
     /* Whether to retry substituting the outputs after building the
-       inputs. */
-    bool retrySubstitution;
+       inputs. This is done in case of an incomplete closure. */
+    bool retrySubstitution = false;
+
+    /* Whether we've retried substitution, in which case we won't try
+       again. */
+    bool retriedSubstitution = false;
 
     /* The derivation stored at drvPath. */
     std::unique_ptr<Derivation> drv;