Update CI to kill ongoing tests on new commit

[OlivierBlanvillain]

For example in #2224 I just amended and forced pushed a commit with, but because Drone is still running the tests for the previous commit that's 15 min of CI time wasted.

To be a good CI citizen I need to open the web interface and cancel every job of the matrix commit by hand...

[felixmulder]

We should probably have the bot do this, I'm not sure drone supports this.

You can also:

 for i in $(seq 6); do drone build stop lampepfl/dotty <build-nbr> $i done

[smarter]

Would be even better if we could cancel all jobs at the same time in the web UI. It's pretty dumb that you cannot do that, but when you restart one job it restarts all the jobs for the current PR :/

[felixmulder]

agreed

[OlivierBlanvillain]

Issue for canceling all jobs of a commit: harness/harness#336

[nicolasstucki]

This is currently happening