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/derivations.hh

@@ -133,7 +133,7 @@ struct Derivation : BasicDerivation
        1. input drv outputs moved to input sources.
 
        2. placeholders replaced with realized input store paths. */
-    BasicDerivation resolve(Store & store);
+    std::optional<BasicDerivation> tryResolve(Store & store);
 
     Derivation() = default;
     Derivation(BasicDerivation && bd) : BasicDerivation(std::move(bd)) { }