Revert "Update the bash syntax of the `minify` script"

This reverts commit 0b240b94e570e08d56346f2859b16b4b9a3f9044.
Make the shell scripts more portable (see https://github.com/contao/core/commit/0b240b94e570e08d56346f2859b16b4b9a3f9044#commitcomment-5984629)