v0.12.1

Fix rename on a real runner installation

GitHub's runner updater points bin and externals at its versioned
directories with absolute symlinks, so moving a runner directory left
both dangling and config.sh could not start. rename failed on the first
runner every time.

If a rename failed on 0.12.0: the pool is already renamed locally, the
first runner is unconfigured, and later runners are still registered
under the old name. Upgrade, delete the old registrations from GitHub's
runner settings, then run runpool reregister .