Add "cancel build" workflow

[ni-jfitzger]

Description of issue

System Tests and Travis CI take a long time to run (somewhere between 45 minutes and an hour and a half maybe?) and sometimes a contributor will push multiple commits to a PR in a short time, causing additional PR checks to be queued. But PR checks for the previous commits remain in the queue and aren't cancelled. This makes it take longer to get test results for the latest commit.

We should automatically cancel testing for previous commits of a PR when a new commit is pushed.
Luckily, there's an Action to help with that: cancel-workflow.
ni/grpc-device actually uses this action, already.

It may take a bit of research to properly use, but it would likely cut down on test time.
We should probably use a later version than ni/grpc-device currently does, so that we don't get node-js12 warnings.