This fetches the notmuch configuration path from the `NOTMUCH_CONFIG`
session variable, if it is set. Specifically, this makes the lieer
service find the configuration generated by the notmuch module.
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'.