[ci] Disabling merge to main at runtime

Hello all, this is quick note that https://github.com/apache/tvm/pull/13475 when landed will change the way CI runs. Currently PRs are merged with origin/main as it exists at the time CI is run. This PR disables that behavior in order to make PR CI runs in Jenkins more reproducible. However, this may lead to more merge-race type bugs where PRs without conflicting code changes end up causing bugs once both are merged to main. Please watch for this over the coming weeks and report a bug on GitHub if you see it.