bug: `compile_qmd_course()` doesn't work if we compile outside of `tempdir`

[ColinFay]

Description

for (i in 1:10){
  write(file = sprintf("test%s.qmd", i),
        sprintf("---
title: 'test%s'
---

## Quarto is cool.", i)
  )
}

> list.files(pattern = "\\.qmd$", full.names = TRUE) |>
   squash::compile_qmd_course(
     output_dir = "mycoursehere",
     output_html = "complete_course.html",
     quiet = FALSE
   ) -> html_file
✔ All qmd rendered.

> browseURL(html_file)
Error in browser(if (encodeIfNeeded) URLencode(url) else url) : 
  File not exists