add recommended pipeline for renv

.gitlab-ci.yml

 image: rocker/verse:4.0.0
+
+variables:
+  RENV_CONFIG_REPOS_OVERRIDE: "http://cran.r-project.org"
+  RENV_PATHS_CACHE: ${CI_PROJECT_DIR}/cache
+  RENV_PATHS_LIBRARY: ${CI_PROJECT_DIR}/renv/library
+
+cache:
+  key: ${CI_JOB_NAME}
+  paths:
+    - ${RENV_PATHS_CACHE}
+    - ${RENV_PATHS_LIBRARY}
+
 before_script:
-  - R -e "install.packages('renv')"
-  - R -e "renv::restore()"
+  - < ... other pre-deploy steps ... >
+  - Rscript -e "if (!requireNamespace('renv', quietly = TRUE)) install.packages('renv')"
+  - Rscript -e "renv::restore()"
+
 pages:
   stage: deploy
   script: