[ci] Assert that Jenkinsfile changes also change the timestamp

[driazati]

Concurrent Jenkinsfile changes that happen in a sequence like:

1. PR 1 makes changes
2. PR 2 makes changes
3. PR 1 merges
4. PR 2 merges

Can create a situation where both PRs pass CI but break main when merged with failures like https://ci.tlcpack.ai/blue/organizations/jenkins/tvm/detail/main/4024/pipeline.

There is a timestamp in the Jenkinsfile that makes it so these PRs will have a merge conflict (in the example above PR 2 must be rebased and run through CI again before it can merge), we should check that any changes to the Jenkinsfile also change this timestamp (so if say PR 2 did not change the timestamp it wouldn't pass PR CI)

cc @Mousius @areusch @gigiblender

[driazati]

This isn't necessary in all cases anymore