Cancel out of date runs

[lcarva]

In a pull request workflow, it is common for the author to modify the changes in the pull request, e.g. push a new commit. When this happens, any previously triggered run is no longer applicable. If any of those are still running, they are mostly wasting resources since their result is thrown out. It would be great if such runs were automatically canceled by PAC.

NOTE: This really only applies to runs from pull request events. Runs from push events, on the other hand, should not be automatically cancelled under the same conditions.

[lcarva]

Somewhat related to #957