updateRustDependencies.sh uses the `realpath` command. macOS doesn't seem to have it, and POSIX doesn't specify it. Maybe we should use something like `(cd $x && pwd -P)` instead for greater portability. (Though older shells and OSes might not have the `-P` option to `pwd`, it is POSIX.) Alternatively, confirm that the current directory is a "reasonable" place to run the script from, and use relative pathnames.