CI: install ggplot2 for tests

Change-Id: I4208c8a36c2fd4e4cb463f4be50d847c24af58d6
diff --git a/ci/install-rkorapclient-ci.yml b/ci/install-rkorapclient-ci.yml
index 93c2234..6e46c8b 100644
--- a/ci/install-rkorapclient-ci.yml
+++ b/ci/install-rkorapclient-ci.yml
@@ -47,7 +47,7 @@
     - start_section installing "Installing RKorAPClient from source"
     - export PATH=/usr/local/sbin:$PATH
     - R -q -e "install.packages('pak', dependencies=TRUE)"
-    - R -q -e "pak::pak('RKorAPClient')"
+    - R -q -e "pak::pak('ggplot2', 'RKorAPClient', dependencies=TRUE)"
     - end_section installing
     - start_section linstalling "Installing RKorAPClient from source"
     - R -q -e 'pak::pak(".")'