`bootc install to-filesystem` does not check for mutable /usr before performing destructive operations

This isn't really a bug, just a behavior that probably needs to be documented or prevented outright.

### To reproduce

1. Have a Fedora Atomic system (with read-only usr)
2. Run this command (from the documentation [here](https://containers.github.io/bootc/bootc-install.html#using-bootc-install-to-existing-root)):

```
sudo podman run --rm --privileged -v /dev:/dev -v /var/lib/containers:/var/lib/containers -v /:/target \
--pid=host --security-opt label=type:unconfined_t \
\
bootc install to-existing-root
```

Observe how your /boot drive will be wiped and then the installation will fail to run `fchmod`, my guess being due to the read-only /usr, aborting the installation.