[R] Fix shellcheck errors in the r/tools/download_dependencies_R.sh #50824

shellcheck r/tools/download_dependencies_R.sh

In r/tools/download_dependencies_R.sh line 48:
  echo 'download.file("'${url}'", "'${out}'", quiet = TRUE)'
                        ^----^ SC2086 (info): Double quote to prevent globbing and word splitting.
                                    ^----^ SC2086 (info): Double quote to prevent globbing and word splitting.

Did you mean:
  echo 'download.file("'"${url}"'", "'"${out}"'", quiet = TRUE)'


In r/tools/download_dependencies_R.sh line 59:
  source ${SOURCE_DIR}/cpp/thirdparty/versions.txt
         ^-----------^ SC2086 (info): Double quote to prevent globbing and word splitting.

Did you mean:
  source "${SOURCE_DIR}"/cpp/thirdparty/versions.txt

For more information:
  https://www.shellcheck.net/wiki/SC2086 -- Double quote to prevent globbing ...