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

[ryanabx]

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):

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 \
             <image> \
             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.

[cgwalters]

This relates to e.g. #137