alert on slack for CI failures after merge to main

[mollykarcher]

What problem does your feature solve?

Both e2e test runs and dry-runs can fail after PRs get merged to main. It's easy to ignore these failures, and then they can stack up over time, making the release particularly difficult.

What would you like to see?

All CI failures on main should post back to slack. It probably makes sense to have a new channel for this, but we could reuse some existing platform alerting channels if we want.

What alternatives are there?

• Block PRs/merges on e2e failures instead
• Run e2e tests periodically, rather than on PRs or merges (System Test: Nightly invocation for soroban test cases system-test#10)

[psheth9]

Request submitted to use Actioner app here which helps with surfacing any kind of status from Github workflow to Slack