Configurable environment variables for passphrases

[radu-matei]

[Original comment:](#80 (comment)

As we think more about how consumers will end up using this as a library rather than a CLI, we should think about making the environment variables for setting passphrases to be configurable.

There are two types of consumers for Signy: end-users, who use the CLI, and other tools that are using the Go packages.
I think there should be a way for both types of users to configure the environment variables used for key passphrases (i.e. SIGNY_ROOT_PASSPHRASE), similarly to how the trust directory is configured.