devtools: add a bash script sending benchmark regression alerts.

Context

Answers this part of #4038 (closed):

this is implemented in https://gitlab.com/nomadic-labs/tezos/-/commits/rafoo@gas_regression_alerts, let's turn this into merge requests:

• add the script creating and sending the reports.

This is already used by the processes detecting benchmark regressions.

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
git fetch
git checkout naih@gas_regression_alerts
git reset --hard origin/naih@gas_regression_alerts
git cherry-pick 16e41f81983c4508dea46f548374f0370b0bf2d2
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 $(opam env) && DISABLE_SLACK_MESSAGING=y /data/redbull/tezos/devtools/benchmarks-tools/watch_regressions.sh
git checkout rafoo@gas_regression_alerts
sed -i 's,$,/,' last_known_dir

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.

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