Recheck path validity after acquiring lock

It might have changed, and in any event this is how the cod used to work
so let's just keep it.

src/libstore/build.cc

@@ -1547,6 +1547,7 @@ void DerivationGoal::tryToBuild()
        omitted, but that would be less efficient.)  Note that since we
        now hold the locks on the output paths, no other process can
        build this derivation, so no further checks are necessary. */
+    checkPathValidity();
     bool allValid = true;
     for (auto & [_, status] : initialOutputs) {
         if (!status.wanted) continue;