Trigger from github comment

[simonbyrne]

Would be nice to be able to trigger builds of specific steps from github comments: either via a specific job name,

@climabot run <jobname>

or specify a yml file in a codeblock

 @climabot run
```
steps:
- command: "mpiexec julia --project foo.jl"
```

(we could use something other than @climabot for this)

Would need to be careful to limit it to authorized users.
Might be able to use https://github.com/buildkite/trigger-pipeline-action

[jakebolewski]

I dont think we can do number 2 looking at the API, but number 1 is possible and seems better as the execution is not arbitrary (needs to go through review)

[ali-ramadhan]

This would be pretty useful! Unfortunately it looks like there's been an open issue about it on Buildkite for years: buildkite/feedback#288

Guess it might have to go through https://github.com/buildkite/trigger-pipeline-action