[WIP] Automatically cancel old PR end to end builds when a new one is triggered

[Kami]

This pull request updates st2_pkg_e2e_test.meta action / workflow to cancel any existing old builds for a particular PR before starting a new one.

This is similar to Travis and Circle CI behavior of auto canceling of old builds when a new build is triggered by pushing changes to a branch. This allows us to speed things up and save resources and utilization on CICD.

To logic to determine old builds is simple - we check Circle CI build number and if that one is < current build number, then we mark that build as old and cancel it.

TODO

• [ ] To ensure there are no stray resources left, we should update cancel action so it also triggers a new cleanup execution (st2ci.st2_pkg_e2e_test_cleanup) for a particular build. That action is idempotent so if it fails we can simply ignore it.

[CLAassistant]

All committers have signed the CLA.