piuparts task doesn't handle conflicting binary packages

piuparts appears to run once with all the generated binaries.

When some of them are conflicting with each others, this fails.

I'm not sure which method would be best, but for comparison:

Salsa CI's approach seems to run piuparts once for each binary package individually, along with a local APT repository to pull potential dependencies from the same new package:
https://salsa.debian.org/lts-team/packages/nginx/-/jobs/7752280

piuparts.debian.org appears to use the same method, except that target packages are already available from sid:
https://piuparts.debian.org/sid/source/n/nginx.html

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