Fix floating CA tests

We will sometimes try to query the outputs of derivations we can't
resolve. That's fine; it just means we don't know what those outputs are
yet.

src/libstore/build.cc

@@ -1460,7 +1460,9 @@ void DerivationGoal::inputsRealised()
             /* We are be able to resolve this derivation based on the
                now-known results of dependencies. If so, we become a stub goal
                aliasing that resolved derivation goal */
-            Derivation drvResolved { fullDrv.resolve(worker.store) };
+            std::optional attempt = fullDrv.tryResolve(worker.store);
+            assert(attempt);
+            Derivation drvResolved { *std::move(attempt) };
 
             auto pathResolved = writeDerivation(worker.store, drvResolved);
             /* Add to memotable to speed up downstream goal's queries with the