[ci] Don't merge before running CI on main

[driazati]

Here Jenkins fetches the latest main and merges code with it to make sure PRs are up to date before running everything:

tvm/ci/jenkins/Prepare.groovy.j2

Lines 16 to 27 in de12486

	// Determine merge commit to use for all stages
	sh (
	script: 'git fetch origin main',
	label: 'Fetch upstream',
	)
	if (upstream_revision == null) {
	upstream_revision = sh(
	script: 'git log -1 FETCH_HEAD --format=\'%H\'',
	label: 'Determine upstream revision',
	returnStdout: true,
	).trim()
	}

This should have a guard so it doesn't happen on main (i.e. when env.BRANCH_NAME is main) and instead upstream_revision is just set to HEAD. Right now the behavior is not correct, since if 2 builds come in in rapid succession, the first one would get merged with the later one and wouldn't be tested independently. This also messes up re-runs of old builds on main since they get merged with the current main.

cc @Mousius @areusch @gigiblender