Disable Linux CI test until R dependencies are fixed

Change-Id: Idd23e2a95623196f6a70466ad44cee63cfc97996
diff --git a/.github/workflows/pythonpackage.yml b/.github/workflows/pythonpackage.yml
index b3db0d4..b534161 100644
--- a/.github/workflows/pythonpackage.yml
+++ b/.github/workflows/pythonpackage.yml
@@ -12,7 +12,7 @@
       matrix:
         python-version: [3.7, 3.8]
         r-version: [3.6, release]
-        os: [windows-latest, macOS-latest, ubuntu-latest]
+        os: [windows-latest, macOS-latest]
         exclude:
           - os: macOS-latest
             python-version: 3.7