lib: Add an "is_system" member to OstreeRepo

lib: Add an "is_system" member to OstreeRepo

This is prep for introducing a fd-relative `ostree_repo_new_at()`.
Previously, `ostree_repo_is_system()` compared `GFile` paths, but
there's a much simpler check we can do first - if this repository
was created via `OstreeSysroot`, it must be a system repo.

Closes: #886
Approved by: jlebon