Introduce more variations in our reproducibility CI tests

Originally created by @intrigeri on #12654 (Redmine)

Currently we test in the exact same VM, and the time varies very little between our two builds. IMO we should introduce as many variations as we can (easily). We have build options for CPU and time variations, that we should use in our second build.

See the Variations introduced when testing Debian packages on tests.r-b.o if we want more inspiration.

Feature Branch: jenkins-jobs:feature/12654-more-build-env-variations-for-RB-tests

Parent Task: tails#5630 (closed)