[ci] fall back to source installs of R packages if binary missing (fixes #3875) (#3876)

* [ci] fall back to source installs of R packages if binary missing

* source option

* changing source policy

* install withr separately

* uncomment CI

.ci/test_r_package.sh

@@ -97,13 +97,18 @@ if [[ $OS_NAME == "macos" ]]; then
     fi
 fi
 
+# hack to fix https://github.com/microsoft/LightGBM/pull/3876#issuecomment-769187766
+if [[ $OS_NAME == "macos" ]]; then
+    Rscript --vanilla -e "install.packages('withr', repos = '${CRAN_MIRROR}', type = 'source', lib = '${R_LIB_PATH}', dependencies = c('Depends', 'Imports', 'LinkingTo'))"
+fi
+
 # Manually install Depends and Imports libraries + 'testthat'
 # to avoid a CI-time dependency on devtools (for devtools::install_deps())
 packages="c('data.table', 'jsonlite', 'Matrix', 'R6', 'testthat')"
 if [[ $OS_NAME == "macos" ]]; then
-    packages+=", type = 'binary'"
+    packages+=", type = 'both'"
 fi
-Rscript --vanilla -e "install.packages(${packages}, repos = '${CRAN_MIRROR}', lib = '${R_LIB_PATH}', dependencies = c('Depends', 'Imports', 'LinkingTo'))" || exit -1
+Rscript --vanilla -e "options(install.packages.compile.from.source = 'both'); install.packages(${packages}, repos = '${CRAN_MIRROR}', lib = '${R_LIB_PATH}', dependencies = c('Depends', 'Imports', 'LinkingTo'))" || exit -1
 
 cd ${BUILD_DIRECTORY}