[Qa-jenkins-scm] race condition found

[jenkins at jenkins.debian.net]

Warning, package db in unstable on amd64 is probably already building elsewhere, exiting.
Please check https://jenkins.debian.net/job/reproducible_builder_theta/6654/ and https://reproducible.debian.net/unstable/amd64/db for a different build.
The race condition was cought while marking the build attempt in the database.