Recreate a mechanism to check knitr PR on a branch of knitr-examples

[cderv]

This was previously possible but removed when Github actions was put in place. https://github.com/yihui/knitr-examples/issues/72

I think it could be easier when we add feature into knitr using PR to be able to add a knitr-examples at the time, and merge the PR when we know the knitr-examples check including the new examples will pass.

Currently this is not possible when using only master.

Could be done locally for our work though so opening this as a reminder to do when there are no other priorities

[cderv]

Or we could use snapshot tests inside knitr directly but that would be considered if ever we switched to use testthat 3 inside knitr, or we develop a similar tooling in testit