lib/sysroot: Add non-failable ostree_sysroot_repo()

lib/sysroot: Add non-failable ostree_sysroot_repo()

Having a failable accessor is annoying, since it's really common
to reference both.  Instead, open the repo once when we load
the sysroot, and provide a non-failable accessor.

This is also prep for `ostree_repo_open_at()`, which collapses the separation
between `ostree_repo_new()` and `ostree_repo_open()`.

Closes: #886
Approved by: jlebon