gitlab-ci: switch to rocker/tidyverse and to pak for install R packages

Ubuntu-stable R version is outdated

Change-Id: Ie454d6c3c26bf27ffa5d0de09ec49fb45ed0874f
diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index 7e33ae3..033402a 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -1,18 +1,17 @@
 include:
   local: ci/install-rkorapclient-ci.yml
 
-test-korapclient-on-latest:
+test-korapclient:
   extends: .install-rkorapclient
-  image: ubuntu:latest
+  image: rocker/tidyverse
   cache:
-    key: rkorapclient-ubuntu-stable
+    key: pykorapclient
     paths:
     - apt-cache/
-    - ccache/
-    - rlib/
+
   script:
     - mkdir -p ~/.config/pip
-    - echo -e "[global]\nbreak-system-packages = true" >> ~/.config/pip/pip.conf 
+    - echo -e "[global]\nbreak-system-packages = true" >> ~/.config/pip/pip.conf
+    - python3 -m pip install pytest
     - python3 -m pip install .
     - python3 -m pytest -v
-