ARROW-7876: [R] Installation fails in the documentation generation image

See build log https://github.com/apache/arrow/runs/452378996#step:5:11843

Closes #6445 from kszucs/ldconfig and squashes the following commits:

242579d <Krisztián Szűcs> run ldconfig after installing the c++ libraries

Authored-by: Krisztián Szűcs <szucs.krisztian@gmail.com>
Signed-off-by: Krisztián Szűcs <szucs.krisztian@gmail.com>

ci/scripts/cpp_build.sh

@@ -129,6 +129,10 @@ time cmake --build . --target install
 
 popd
 
+if [ -x "$(command -v ldconfig)" ]; then
+  ldconfig
+fi
+
 if [ "${ARROW_USE_CCACHE}" == "ON" ]; then
     echo -e "===\n=== ccache statistics after build\n==="
     ccache -s