Fix copy and clean-up problems on immutable OS

[eline-anslot]

The sass library create problems when used on an immutable system like NixOS, see GitHub issue for more context. The same problem has been solved in officer package in an identical manner.

I do not think it is possible to add unit tests in the tests/testthat directory. Note that the style guideline returns a 404 error.

• Run devtools::check() (or, equivalently, click on Build->Check Package in the RStudio IDE) to make sure your change did not add any messages, warnings, or errors.

  • Note there is a decent chance that some tests were already failing before your changes. Just make sure you haven't introduced any new ones.

• Add an entry to NEWS.md concisely describing what was changed.

[CLAassistant]

All committers have signed the CLA.