find-remotes: Don't require write access to repo unless pulling

find-remotes: Don't require write access to repo unless pulling

ostree find-remotes is a read-only operation that searches for remotes
which have a given collection-ref. It should not require write access
to the repository. Only when --pull is used does the command actually
write to the repo.

Move the ostree_ensure_repo_writable() check to just before the pull
phase so that read-only find-remotes works on read-only repositories.

Closes #3585