We used to always set the sysroot.readonly setting to true, but this
never worked because of a bug in osbuild [1].
The bug is now fixed and the RHEL and CentOS edge-raw images are crated
with sysroot.readonly = true, and the images aren't booting.
Fixing the option to false. This changes the manifests, but not the
generated images because of the change in osbuild.
If sysroot is meant to be readonly, we will change it in a future
update.
[1] https://github.com/osbuild/osbuild/pull/1129
