The "Keychain" facility for Linux should read environmental variables

[masonmark]

To facilitate getting a working build on Linux, a rudimentary Keychain was added. But unlike on the macOS & iOS environments, where the OS-provided secure storage mechanisms are used, the Linux "Keychain" just writes plaintext blobs to the filesystem.

This is obviously inadequate and temporary; a minimally viable solution would be to extend Keychain to be able to read credentials from environment variables.