updated validate-ctv GHA with improved performance

[zeileis]

Hi Sam @boshek , we have updated our cran-task-views/ctv/validate-ctv GitHub action using the rocker/r2u container suggested by Dirk inorder to speed up running the action. Also, we have streamlined the R code of the action a bit.

For the action to work correctly a small addition in the validate-ctv workflow of your repository is necessary, namely declaring the rocker/r2u container. This is what this PR provides. Let us know if anything is unclear.

[zeileis]

The new improved GHA found two unavailable links:

• https://owi.usgs.gov/R/
  This does not seem to work anymore. Instead of the usgs.gov link I've now used the link of the GitHub org which still exists but hasn't been active recently. I would keep it in the links, though, in case it becomes more active again.

• https://R-Forge.R-project.org/projects/dream
  This seems to be a temporary problem because R-Forge can currently not be reached. I think everything should be fine again soon

[boshek]

Thanks @zeileis. This is amazing work! I ended up dropping dream from the task view as there is now another package on CRAN of the same name and the R forge link still doesn't work. Already this is paying dividends!

[zeileis]

Thanks Sam @boshek !

Regarding dream and R-Forge: The R-Forge server was under attack from hackers this week, hence https access first was very slow and then turned off. I hope the admin team at WU Wien will sort out the situation very soon.

So it's not necessary, yet, to drop the dream project.