deploy: Don't fail if loading composefs configuration fails due to missing keys

deploy: Don't fail if loading composefs configuration fails due to missing keys

When we load the configuration during deploy we don't need to actually
use the keys, so avoid loading them. This fixes an issue we had where
this broke the initial deploy becasue of a failure to load the key. In
our case it fails because the code looks for the config file in the
deploy dir, but then for the binding key in the real root.

However, even if it were to look for the key in the deploy dir I don't
think it necessarily has to be in the rootfs, it could be only in the
initrd.

This fixes https://github.com/ostreedev/ostree/issues/3188