Adding a performance regression breaks perf-benchcomp CI workflow

[zhassan-aws]

Adding any test to the performance regressions breaks the benchcomp CI worflow. Here is one example PR that I opened to reproduce this issue:

#2422

Here is the CI run results:

https://github.com/model-checking/kani/actions/runs/4866466752/jobs/8678021607?pr=2422

I first noticed this in the s2n-quic update PR:

#2422

[zhassan-aws]

CC @karkhaz

[zhassan-aws]

The CI workflow should compare the two versions (pre/post-PR commits) on the same set of benchmarks. In other words, it should only use the Kani binaries from the pre-PR commit (but not the benchmarks). The benchmarks in the PR should be used for comparison. This is to make sure the comparison is apples-to-apples as the PR may have modified the benchmarks, or added/removed some of them.