lib: Stop using old `ostree_sysroot_get_repo()` API

lib: Stop using old `ostree_sysroot_get_repo()` API

It's falliable, and in one place we were actually ignoring
the error and leaving a `NULL` repo object which is just a trap
for people coming along later since it's rarely nullable.

Quite a while ago we switched to loading the repo at the same time
as the sysroot; convert callers in the library to use this infallible
accessor.

Prep for another patch which will use the repo object.