watch_regressions improvements

Context

Implement 2 features when watching regressions:

• filter old benchmarks with the FIRST_DIR environment variable;
• compare the latest run with a reference one using the REF_DIR environment variable.

Manually testing the MR

On 163.172.34.197, with a sudoer access, let's play the Cron script with Slack messages disabled, and as if the latest benchmarks run hasn't been treated:

sudo su redbull
cd /data/redbull/tezos
aws s3 ls s3://snoop-playground/mclaren/inference_csvs/snoop_results/ | grep PRE | tail -n 2 | head -n 1 | sed 's/ *PRE //' | sed 's,/$,,' > last_known_dir
eval $(opam env) && DISABLE_SLACK_MESSAGING=y FIRST_DIR=_snoop_20221118_2000_a5732235ef4e9820897a93ed8b9fcafb4fd44180 REF_DIR=_snoop_00_post-gas_for_M_based_on_20230110_2000_v15.0-rc1-2705-g1292297f9a /data/redbull/tezos/devtools/benchmarks-tools/watch_regressions.sh

Then, compare the alerts in file /data/redbull/tezos/alerts with the latest ones reported on Slack by the bot. They should be the same, except for a few cosmetic changes.

Checklist

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• Select as Assignee the next person who should take action on that MR