Defaults Helper Does Not Follow New File Name Format

[shannona]

Per #171, Bitcoin Core 23.0 introduced a new file name format that differentiates between arm64 (M1) and x86_64 (Intel). The default values for the Helpers in Helpers/Defaults.Swift do not recognize this. The uname() command in Swift can be used to set this correctly, with the new file name format being one of:

bitcoin-23.0-arm64-apple-darwin.tar.gz
bitcoin-23.0-x86_64-apple-darwin.tar.gz

However, that may not do the right thing if the program is running in Rosetta on an M1, where Bitcoin Core itself preferably runs as the arm64 binary.