updateRustDependencies.sh shouldn't use realpath

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.