nixos: initrd/luks: change passphrases handling

Also reuse common cryptsetup invocation subexpressions.

- Passphrase reading is done via the shell now, not by cryptsetup.
  This way the same passphrase can be reused between cryptsetup
  invocations, which this module now tries to do by default (can be
  disabled).
- Number of retries is now infinity, it makes no sense to make users
  reboot when they fail to type in their passphrase.