Always check for build availability

Now that we don't assume the local store anymore, but use the
BINARY_CACHE_STORE which is effectively `openStore(store_uri)`, it
doesn't really matter where the store is.

Also, this doesn't seem too expensive for at most 5 build outputs.

src/lib/Hydra/Controller/Build.pm

@@ -79,9 +79,7 @@ sub build_GET {
     # XXX: If the derivation is content-addressed then this will always return
     # false because `$_->path` will be empty
     $c->stash->{available} =
-        $c->stash->{isLocalStore}
-        ? all { $_->path && $BINARY_CACHE_STORE->isValidPath($_->path) } $build->buildoutputs->all
-        : 1;
+        all { $_->path && $BINARY_CACHE_STORE->isValidPath($_->path) } $build->buildoutputs->all;
     $c->stash->{drvAvailable} = $MACHINE_LOCAL_STORE->isValidPath($build->drvpath);
 
     if ($build->finished && $build->iscachedbuild) {