child documents paths as R code cause validation error

Hi,

I'm having some issues with a more recent version of sandpaper where the deploy step is failing in Github.

Local builds of the site build without issue.

**The problem:**

I use r eval statemets within Rmd documents to dynamically include content depending on a configuration for the lesson I want to publish. This was working previously but there is now an issue I think in the way the deploy function finds and copies content. There don't appear to be any logs or useful output in the Github actions build process because the R output is limited in the deploy step.

I have created a MWE that demonstrates the failing build process.

https://github.com/trhallam/workbench-mwe/actions/runs/6392622732/job/17350256259