Update touchstone comment behaviour

[VisruthSK]

Summary

Make touchstone comments update the same touchstone comment like codecov instead of making a new comment every run. Action copied from loo.

Copyright and Licensing

By submitting this pull request, the copyright holder is agreeing to license the submitted work under the following licenses:

• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
• Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)

[github-actions]

This is how benchmark results would change (along with a 95% confidence interval in relative change) if 5cd6dea is merged into master:

• ✔️as_draws_array: 160ms -> 160ms [-1.07%, +0.67%]
• ✔️as_draws_df: 81.4ms -> 80.5ms [-2.92%, +0.57%]
• ✔️as_draws_list: 171ms -> 172ms [-0.81%, +1.07%]
• ✔️as_draws_matrix: 25ms -> 25.1ms [-0.28%, +1.26%]
• ✔️as_draws_rvars: 125ms -> 126ms [-0.51%, +1.98%]
• ❗🐌summarise_draws_100_variables: 766ms -> 797ms [+3.29%, +4.71%]
• ✔️summarise_draws_10_variables: 86.1ms -> 86ms [-0.97%, +0.75%]
  Further explanation regarding interpretation and methodology can be found in the documentation.