Cancel previous running workflows

[mzgubic]

I don't know anything about CI, so this might be very wrong - just testing it out

Closes #426

[devmotion]

I haven't used it anywhere (just opened a PR to KernelFunctions where we had an open issue about canceling workflows) but I think it could be helpful to configuration in https://github.com/styfle/cancel-workflow-action#advanced-pull-requests-from-forks such that it also works for PRs from forks.

[mzgubic]

Could we disable email notification when the workflow is cancelled rather than when it fails?

[mzgubic]

By default, only the workflows started before this action are cancelled. This flag cancels all but the latest workflow, which is helpful if the pipeline is saturated

[oxinabox]

Hmm this seems to have resulted in everything being cancelled because of VersionVigilante failing.
We don't want everything cancelled when anything fails, since Nightly fails all the time,

[mzgubic]

Oh no I cancelled manually because I didn't want to block other pipelines, sorry!

This only runs when merged to master (as per JuliaGaussianProcesses/KernelFunctions.jl#292)

[oxinabox]

OK, lets give this a try

[mzgubic]

Did not seem to have worked, i.e. this PR still ran two full pipelines:
#433