reproducibly_build_Tails_ISO_* Jenkins jobs are buggy when building from a tag

Originally created by @intrigeri on #12681 (Redmine)

We get two ISOs called exactly the same, so the 2nd one overwrites the 1st one and there’s nothing to compare. E.g. https://jenkins.tails.boum.org/job/reproducibly_build_Tails_ISO_testing/46/console.

Too bad, release time is perhaps one of the times when those results are most useful.

This should be trivial to fix while working on #12633 (closed).

Parent Task: tails#5630 (closed)