Reduce comment noise: produce only a single comment per artifact

[rtyley]

At the moment, actions-riff-raff automatically produces one PR comment per riff-raff artifact - this gets noisy on projects with many artifacts - eg Ophan, with 8 comments, or editorial-tools-platform, with 12 comments.

We don't want developers to have to hunt for non-bot comments on PRs - the human-written comments on PRs are generally the most important ones, and shouldn't be lost in a swarm of bot comments! Consequently, we should restrict the automatic commenting by actions-riff-raff to one comment per PR - with that comment updated as new artifacts (and ideally commits) arrive.

See also some previous comments about this issue:

• fix(commenting): Update all comments created by actions-riff-raff #95 (comment) - from @twrichards
• feat!: Add comment to PR #85 (comment) - from @rtyley

[akash1810]

I think producing one comment is likely the best behaviour, however it's worth mentioning an alternative that exists today would be to split across GitHub workflow files. The guardian/deploy-tools-platform repository does this, for example. The trade-off is increased complexity to the workflow files, with the setting of paths within the on directive.

[akash1810]

Another example of noisy comments - guardian/support-service-lambdas#2587.

[akash1810]

#184 is a pragmatic attempt to improve the DX here. Its only pragmatic; the ideal would be to produce exactly one comment with all the links.