DEV: Avoid cancelling in-progress tests for `main` (PR #15101)

Followup to DEV: Cancel in-progress test runs on PRs (#15096) · discourse/discourse@78830d1 · GitHub, we only want to cancel PR runs, not main branch runs.

GitHub

The title of this pull request changed from “DEV: Avoid cancelling in-progress tests for branches” to "DEV: Avoid cancelling in-progress tests formain

The title of this pull request changed from “DEV: Avoid cancelling in-progress tests formain” to "DEV: Avoid cancelling in-progress tests for main