GH-46673: [CI][R][Docs] Accept empty INSTALL_ARGS again (#46682)

### Rationale for this change

This PR fixes the bug introduced in #46527.

If the environment variable `INSTALL_ARGS` doesn't set,
the R command should execute,
`CMD INSTALL arrow*tar.gz`. 

After #46527 change, It executed  
`CMD INSTALL '' arrow*tar.gz`

### What changes are included in this PR?

Split `INSTALL_ARGS` to `R_INSTALL_ARGS` as array and use `"${R_INSTALL_ARGS[@]}"`.

### Are these changes tested?

Yes.

### Are there any user-facing changes?

No.
* GitHub Issue: #46673

Lead-authored-by: Hiroyuki Sato <[email protected]>
Co-authored-by: Sutou Kouhei <[email protected]>
Signed-off-by: Sutou Kouhei <[email protected]>

Author: hiroyuki-sato

ci/scripts/r_build.sh

@@ -24,6 +24,11 @@ build_dir=${2}
 
 : "${BUILD_DOCS_R:=OFF}"
 
+R_INSTALL_ARGS=()
+for arg in ${INSTALL_ARGS:-}; do
+  R_INSTALL_ARGS+=("${arg}")
+done
+
 # https://github.com/apache/arrow/issues/41429
 # TODO: We want to out-of-source build. This is a workaround. We copy
 # all needed files to the build directory from the source directory
@@ -42,7 +47,7 @@ fi
 ${SUDO} \
   env \
     PKG_CONFIG_PATH="${ARROW_HOME}/lib/pkgconfig:${PKG_CONFIG_PATH}" \
-      "${R_BIN}" CMD INSTALL "${INSTALL_ARGS}" arrow*.tar.gz
+      "${R_BIN}" CMD INSTALL "${R_INSTALL_ARGS[@]}" arrow*.tar.gz
 
 if [ "${BUILD_DOCS_R}" == "ON" ]; then
   ${R_BIN} -e "pkgdown::build_site(install = FALSE)"