feat: Make the parallel rendering robust

[ColinFay]

Description

Right now the parallel rendering isn't reliable and returns error because it doesn't find the mediabag folder.

future::plan(future::multisession, workers = 8)

for (i in 1:100){
  write(file = sprintf("test%s.qmd", i),
        sprintf("---
title: 'test%s'
---

## Quarto is cool", i)
  )
}

list.files(pattern = "\\.qmd$", full.names = TRUE, recursive = TRUE) |>
  squash::compile_qmd_course(
    output_dir = tempdir(),
    output_html = "complete_course.html"
  )

ℹ {future} is using future::plan(future::multisession, workers = 8), to modify this use `future::plan()`
✖ Failed to render ./test18.qmd, cleaning and existing
✖ Failed to render ./test40.qmd, cleaning and existing
✖ Failed to render ./test45.qmd, cleaning and existing
✖ Failed to render ./test47.qmd, cleaning and existing
✖ Failed to render ./test54.qmd, cleaning and existing
✖ Failed to render ./test74.qmd, cleaning and existing
✖ Failed to render ./test75.qmd, cleaning and existing
✖ Failed to render ./test76.qmd, cleaning and existing
✖ Failed to render ./test78.qmd, cleaning and existing
✖ Failed to render ./test87.qmd, cleaning and existing
✖ Failed to render ./test95.qmd, cleaning and existing
Error in squash::compile_qmd_course(list.files(pattern = "\\.qmd$", full.names = TRUE,  : 
  Failed to render all qmd files.

Diagnostic is that the folder is created by one process, listed by the second, then deleted by the first, and unreachable by the second