Let users customise .Rprofile

[bschilder]

It may be useful for users to be able to supply a customised .Rprofile in some cases, eg:
#65 (comment)

However this step in the rworkflows actions currently overwrites any existing .Rprofile on Linux:

rworkflows/action.yml

Line 203 in 83cabfd

echo ".libPaths('/__w/_temp/Library')" > ~/.Rprofile

Instead, I think I can change this to just append the .Rprofile using >> instead of >
https://unix.stackexchange.com/a/64376

I confirmed that >> doesn't cause an error when no such file yet exists.

More info on .Rprofile:
https://support.posit.co/hc/en-us/articles/360047157094-Managing-R-with-Rprofile-Renviron-Rprofile-site-Renviron-site-rsession-conf-and-repos-conf