Allows user to have more-diffable encrypted .xcconfig
files to better show what keys have changed in a diff by encrypting each value individually.
For now, the nonce is zeroes for repeatability / idempotency. A nonce header would require some additional processing, but would offer additional protection.
(Using a dynamic nonce will result in a differing encrypted value every time the script is run, which is undesirable for diff
purposes.)