Update certora.yml to pin the cli version

[mmv08]

The certora-cli v5 release on 15th Nov has some breaking changes that can lead to CI runs failing. To avoid these failures, we must change run: pip install certora-cli to run: pip3 install certora-cli==4.13.1 in certora.yml under .github/workflows. This will ensure there are no failures due to the new release.

EDIT: For some reason, the coveralls action started failing with an error. Example run: https://github.com/safe-global/safe-contracts/actions/runs/6811949628/job/18526604192?pr=696

I updated our coveralls action based on an example for parallel actions at https://github.com/marketplace/actions/coveralls-github-action and https://docs.coveralls.io/parallel-builds (was linked in the failed run)

[coveralls]

Pull Request Test Coverage Report for Build 6875977664

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.3%) to 94.307%

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Files with Coverage Reduction	New Missed Lines	%
contracts/examples/guards/OnlyOwnersGuard.sol	1	71.43%
<!--	Total:	1		-->

Totals
Change from base Build 6861389476:	-0.3%
Covered Lines:	400
Relevant Lines:	409

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💛 - Coveralls