Fix /opt to be a directory

[cgwalters]

Tracking this here from ostreedev/ostree-rs-ext#607

This currently wants a patch to rpm-ostree...basically coreos/rpm-ostree@7857feb special cased root.transient = true, which we were going to ship but then I got talked down (correctly, probably) to just using composefs, but not transient root.

Except as determined there we need an opt-in to configure the symlinks, which we should then flip in this repository as we always want to have the container-oriented flow.

[cgwalters]

#425
is part of this, but we still need to fix /opt to be a directory here. It's quite unfortunate because the rpm-ostree code really just doesn't let external things do this right now.

What I was looking at is adding an "execute arbitrary code afterwards" flow. But the real fix of course is to go to a full container-native build system.

[cgwalters]

ostreedev/ostree#3214 will also help here as it will fix being able to do this in derived layers.

[cgwalters]

PR in #438