lint: `/ostree` symlink required ➡️ thinking about composers-native signaling

Right now `bootc container lint` checks for `/ostree` being a symlink, even if `/sysroot/ostree` isn't present. This is mildly annoying for people doing composefs-native systems.

However: Digging into this, I think what we really want is a more explicit opt-in to a composefs-backed install beyond just "has sealed UKI". We have three knobs:

1. compile time default (in fact I'd like to support compiling bootc without ostree at all, that's a whole other thing)
2. install config
3. Detect `/usr/lib/composefs/setup-root-conf.toml` (even if empty)

Of these, I think 3 is immediately actionable and matches the requirement for `ostree-prepare-root.conf` at least.