Automerge: [Github][CI] Make the new premerge system fail jobs on errors

This patch makes the new premerge system report failures when the build
errors out. We were previously not doing this to not notify people on
failures as we were testing out the infra.

This works towards making the new premerge system canonical and the
deprecation of the old system.

A launch announcement on Discourse will accompany this commit.

Reviewers: cmtice, tstellar, joker-eph, Keenuts, dschuff, lnihlen, gburgessiv

Reviewed By: tstellar, joker-eph, Keenuts

Pull Request: llvm/llvm-project#139359

Author: boomanaiden154