git creds cache on linux

[mdsumner]

Thanks for this awsome page, I keep referring back to it.

https://carpentries.github.io/sandpaper-docs/github-pat.html

But, this blog has just been shown to me that explains how the creds time out very quickly on linux, and how to fix:

https://blog.djnavarro.net/posts/2021-08-08_git-credential-helpers/#unless-youre-on-linux

(It's been bugging me and I kept going back to ssh/.Renviron setting ...)

Please link to this or include a note. I'd be happy to do this as a PR, I just need to note it somewhere for now. Thanks!!