Use same perf tests for benchcomp in CI

[qinheping]

Description of changes:

The current benchcomp compares the two versions (pre/post-PR commits) of Kani on the corresponding two versions of benchmarks. It make no sense to compare their performance if the commit modified the performance benchmarks.

In this PR, we copy the post-PR benchmarks to the pre-PR folder to make sure benchcomp always run on the same set of benchmarks.

Here is the CI run result with this commit
https://github.com/qinheping/kani/actions/runs/4879281724/jobs/8705705561?pr=3
comparing to the current workflow
https://github.com/model-checking/kani/actions/runs/4866466752/jobs/8678021607?pr=2422

Resolved issues:

Resolves #2424

Related RFC:

Optional #ISSUE-NUMBER.

Call-outs:

Testing:

• How is this change tested?

• Is this a refactor change?

Checklist

• Each commit message has a non-empty body, explaining why the change was made
• Methods or procedures are documented
• Regression or unit tests are included, or existing tests cover the modified code
• My PR is restricted to a single feature or bugfix

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 and MIT licenses.

[celinval]

Thanks for fixing this.

@karkhaz do you have any plan to address this issue as part of the tool or the configuration script?