install.sh case $uname_target: additional err msg

[bnonni]

add additional err msg to build from source for users who's arch falls outside of the list (e.g. aarch64-Linux). Should help address https://github.com/casey/ord/issues/1628 and mitigate future issues on the topic.

case $uname_target in
    arm64-Darwin) target=aarch64-apple-darwin;;
    x86_64-Darwin) target=x86_64-apple-darwin;;
    x86_64-Linux) target=x86_64-unknown-linux-gnu;;
    *)
      err 'Could not determine target from output of `uname -m`-`uname -s`, please use `--target`:' $uname_target
      err 'Please try building from source: https://github.com/casey/ord#building'
    ;;
  esac