services: Fix missing environment variables for user services

We add an additional 'core' user service (i.e. one that gets loaded
before all others by turnstile) which populates the TURNSTILE_ENV_DIR
with all manner of custom set env vars that are important for other
applications.

Most importantly, this sets up the XDG directory compliance for
applications either managed by turnstile or applications started through
turnstile on my system. So, for example `pass` knows to search for its
database in `XDG_DATA_HOME` and river knows to search for binaries in a
PATH which has been prefixed with my custom user binary location.

services/sv/loadenv/log/run

@@ -0,0 +1,3 @@
+#!/usr/bin/env sh
+
+exec vlogger -t loadenv -p daemon