make "R CMD CRAN incoming" a manual and optional check

[jacobvjk]

the GH action R CMD CRAN incoming is producing noise that appears irrelevant to PRs that are not preparing a submission to CRAN (see #379 ). We therefore want to make it an action that has to be triggered manually and that we will use as a preparatory step for CRAN releases. It should not be blocking PRs to main that are not preparing for a release

[jacobvjk]

fyi @jdhoffa @cjyetman

[cjyetman]

@jacobvjk sorry to keep dragging this topic out, but I was considering the possibility of making this CI run automatically and always on PRs but fixing it so that it never fails/errors, but that it only posts a comment to the issue thread if it finds CRAN incoming notes. Might that be preferable?

[jacobvjk]

probably better, because we won't have to remember triggering it manually. I am okay with that