bullseye piuparts test runner environment is missing the bullseye-updates apt repository

The spamassassin debian/bullseye-backports branch sets RELEASE: 'bullseye' in salsa-ci.yml in order to execute the pipeline in a bullseye environment. Currently the piuparts test is failing (see https://salsa.debian.org/debian/spamassassin/-/jobs/3631846) due to a mismatch between libc6 in the environment and libc6-dev available in the repositories. The current libc6 packages are available in the bullseye-updates repository, and it seems like that repository is either missing or stale in the runner environment. This seems to be specific to the piuparts test, as the libc6-dev packages is successfully pulled from bullseye-updates in other jobs (e.g. test-build-any)

Dependency installation fails with:

  The following packages have unmet dependencies:
   libc6-dev : Depends: libc6 (= 2.31-13+deb11u4) but 2.31-13+deb11u5 is to be installed
  E: Unable to correct problems, you have held broken packages.

Whereas that package is actually available:

root@57148642d6d0:/# apt policy libc6-dev
libc6-dev:
  Installed: (none)
  Candidate: 2.31-13+deb11u5
  Version table:
     2.31-13+deb11u5 500
        500 http://deb.debian.org/debian bullseye-updates/main amd64 Packages
     2.31-13+deb11u4 500
        500 http://deb.debian.org/debian bullseye/main amd64 Packages