Failing `main` build notification on Slack

[LesnyRumcajs]

Issue summary So far if an issue appears on the main branch we don't get notified - basically, someone needs to look at the repo main page to notice it. This possibly makes the main branch red for an extended period of time.

Example:

1. PR opened on Monday, all checks passed.
2. On Tuesday there's a toolchain update that makes clippy angry about Forest's code.
3. On Wednesday the PR gets reviewed and merged (because it was green). The checks don't pass anymore, we get a red Build failed badge and everyone look at Forest with contempt.
4. Next Monday someone opens a PR and has to fix random linter issues or someone notices it and fixes it sometime after.

Surely we can do better!

Task summary The idea is to make a Slack notification on failed main build - this way everyone gets immediately and the issue can be resolved as soon as possible.

We could use this or something similar. The only thing we would need is a generated slack webhook token which we would put in repo secrets.

Acceptance Criteria Our filecoin-notif channel gets a notification on main build fiasco.

Other information and links