Fix incorrect download locking.

[reitermarkus]

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Saw this error here: https://github.com/reitermarkus/dotfiles/actions/runs/11988870070/job/33424635021#step:10:7165

When a download is interrupted at exactly the right time, that is after the LockFile object was created but before the lock file is actually created on disk, the download_lock&.path&.unlink step will fail.

This change ensures the lock/unlock methods themselves are never interrupted, and moves the unlink step into the unlock method as well, so we only remove the lock file if we actually hold the lock.

Also fixes another race condition which may happen if one process deletes a lock file after another already opened it.