Use the `os-ostree_sysroot_init_osname` API instead of the `os-init` libostree CLI

[omertuc]

See discussion

https://github.com/containers/bootc/blob/d17e471eaa1da722bed005dfa820eac76aaf876e/lib/src/install.rs#L603-L606

Originally posted by @omertuc in #780