Update GitHub Actions to avoid caching issue

[davidkretch]

We get the following error in our Ubuntu checks, apparently due to issue https://github.com/r-lib/actions/issues/163.

unable to load shared object '/home/runner/work/_temp/Library/stringi/libs/stringi.so':
  libicui18n.so.66: cannot open shared object file: No such file or directory

This is apparently resolved in newer versions of the R workflow examples. I manually cleared our caches to temporarily fix the problem.