Snapshot-ci slack notification

[clementleclercRTE]

Please check if the PR fulfills these requirements

• The commit message follows our guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

Does this PR already have an issue describing the problem?

What kind of change does this PR introduce?

• Added Slack notifications for failed snapshot-CI workflow executions (requires adding a SLACK_WEBHOOK_URL GitHub secret)
• Snapshot-CI workflow continues even after build failures

What is the current behavior?

What is the new behavior (if this is a feature change)?

Does this PR introduce a breaking change or deprecate an API?

• Yes
• No

If yes, please check if the following requirements are fulfilled

• The Breaking Change or Deprecated label has been added
• The migration steps are described in the following section

What changes might users need to make in their application due to this PR? (migration steps)

Slack notifications requires adding a SLACK_WEBHOOK_URL GitHub secret

Other information: