Ability to add text to include when comment posted with checks fallback

[oxinabox]

I am currently trying to workout best way to deal with JuliaDiff/ChainRulesCore.jl#489
where the suggester action doesn't have the permissions needed to make a suggestion to a PR from a fork.

One thing that might help is if I could define a string so that every comment make using the check fallback (that doesn't allow the "apply suggestion") includes that string, which would be something like:

To automatically format the whole repo (and thus fix this and other issues) run ...