Relock wait for build slot goals

When we don’t have enough free job slots to run a goal, we put it in
the waitForBuildSlot list & unlock its output locks. This will
continue from where we left off (tryLocalBuild). However, we need the
locks to get reacquired when/if the goal ever restarts. So, we need to
send it back through tryToBuild to get reqacquire those locks.

I think this bug was introduced in
https://github.com/NixOS/nix/pull/4570. It leads to some builds
starting without proper locks.

src/libstore/build/local-derivation-goal.cc

@@ -153,6 +153,7 @@ void LocalDerivationGoal::killChild()
 void LocalDerivationGoal::tryLocalBuild() {
     unsigned int curBuilds = worker.getNrLocalBuilds();
     if (curBuilds >= settings.maxBuildJobs) {
+        state = &DerivationGoal::tryToBuild;
         worker.waitForBuildSlot(shared_from_this());
         outputLocks.unlock();
         return;