reprotest fails when building against experimental with dependencies in experimental

See here https://salsa.debian.org/qt-kde-team/extras/kaidan/-/pipelines/203710 for an example of failure.

I added RELEASE: 'experimental' to the pipeline variables but the reprotest didn’t correctly resolve dependencies from experimental.

From the logs:

Using Docker executor with image registry.salsa.debian.org/salsa-ci-team/pipeline/reprotest:latest ...
[…]
$ apt-get update
Get:1 http://deb.debian.org/debian sid InRelease [150 kB]
Get:2 http://deb.debian.org/debian sid/main amd64 Packages [8530 kB]

mentioned in issue #236

marked this issue as related to #236

marked this issue as related to #57

Duplicate of #57

mentioned in commit ruby-team/vagrant@c95bc620

mentioned in issue #434