fix: support arm64 macs in install-sass

get_arch only handled x86_64, so arm64 machines hit "unsupported
uname: arm64" before any download was attempted. Also add a local
get_os override: dart-sass names its macOS assets "macos-*", but the
shared get_os helper returns "darwin", which 404s.

dot_local/bin/executable_install-sass

@@ -19,8 +19,18 @@ main() (
 get_arch () (
 	case $(uname -m) in
 		x86_64*)    echo x64 ;;
+		arm64*)     echo arm64 ;;
+		aarch64*)   echo arm64 ;;
 		*)          >&2 echo "unsupported uname: $(uname -m)"; exit 1 ;;
 	esac
 )
 
+get_os () (
+	case $(uname -s) in
+		Linux*)     echo linux ;;
+		Darwin*)    echo macos ;;
+		*)          >&2 echo "unsupported os: $(uname -s)"; exit 1 ;;
+	esac
+)
+
 main "${@}"