Before the XDG variables would be set from the user's environment, if
available. This would break some tests.
With this change the tests should be fully deterministic.
Fixes#1190
Add 'services.lieer', which generates systemd timer and service units
to synchronize a Gmail account with lieer. Per-account configuration
lives in 'accounts.email.accounts.<name>.lieer.sync'.
This replaces some derivation outputs by simple strings rather than
full Nix store paths. This removes the need to download the whole
derivation when all we need is a static string.
