lib/repo: Use correct name for tmpdir lockfile

lib/repo: Use correct name for tmpdir lockfile

Such an evil bug 🙈. I was just reading an strace trying to figure out what was
going on, and noticed we had the `XXXXXX` in the lockfile name. It was only
after that I realized that that this might *be* the cause of the skopeo issue.

This is another case where we definitely need more test coverage of things that
actually use the API multiple times in process; might look at dusting off the
work for the rpm-ostree test.

Closes: https://github.com/ostreedev/ostree/issues/1196
Closes: #1204
Approved by: jlebon