Racy rsync failures releasing multiple packages at the same time

I sometimes (e.g. with kernel rebuilds) run pkgctl release for multiple packages at the same time.

Rarely, this produces a message like the following and fails to upload a file:

Unable to create partial-dir for deepin-anything-arch-6.1.2-123-x86_64.pkg.tar.zst.sig -- discarding completed file.

I guess this happens when two invocation of rsync both determine the partial dir does not already exist and attempt to create it. If so, this is connected to the --partial-dir and --delay-updates rsync options that were added in 5780ba0e.

pkgctl does not exit with an error.