mmdebstrap task randomly fails finding a keyring

In https://debusine.debian.net/debusine/System/work-request/70833/, a mmdebstrap task failed finding a keyring. Retrying, made it work. Another example is https://debusine.debian.net/debusine/System/work-request/70845/. I retried many of the tasks and the ARM ones seem to succeed eventually whereas the X86 ones seem to continue failing. It could be load-dependent, but it definitely looks like a race condition of sorts.