CI tests: work around caching problem

Change-Id: I8a46aa756bb92cd8a7f37dca6021e8dc903fad93
diff --git a/.github/workflows/pythonpackage.yml b/.github/workflows/pythonpackage.yml
index 7c09d55..75b59c3 100644
--- a/.github/workflows/pythonpackage.yml
+++ b/.github/workflows/pythonpackage.yml
@@ -16,7 +16,7 @@
           - os: macOS-latest
             python-version: 3.7
           - os: macOS-latest
-            r-version: 3.6
+            r-version: 4.0
           - os: windows-latest
             python-version: 3.7
           - os: windows-latest