Propagate interrupts to remote commands

Propagate interrupts to remote commands

This appears to be already fixed in master (`Buildstep.runCommand` has this same fix).  I opted to only patch `Source` instead of `LoggingBuildstep` or `Buildstep` on the off chance that this fix might break someone.  I'm happy to make that change to, if that is deemed to be the better fix.

`trial buildbot.test` is how I ensured I didn't break anything with this change.