Unlock output paths when a derivation is already built

Without this change, nix build processes will not drop the locks for derivation goals
which have already been built by another process when the current process gets
round to building them. This means the locks are held until the process
terminates.

If there are other nix build processes in a similar state, they will also try to
acquire the same locks when they try to build the same derivation, and so will
wait until the lock holder terminates (which might be a very long time if it has
a lot to build). In some pathological cases, those processes might be holding
their own locks on other derivations due to the same issue, and this can lead to
deadlock.

Resolves #6468

src/libstore/build/derivation-goal.cc

@@ -690,6 +690,7 @@ void DerivationGoal::tryToBuild()
     if (buildMode != bmCheck && allValid) {
         debug("skipping build of derivation '%s', someone beat us to it", worker.store.printStorePath(drvPath));
         outputLocks.setDeletion(true);
+        outputLocks.unlock();
         done(BuildResult::AlreadyValid, std::move(validOutputs));
         return;
     }