[nixos] Fix race condition with user units (#2286)

One of the things managed by the `home-manager-<username>` unit is the systemd
user directory `.config/systemd/user`.  However, this directory needs to be in
place completely before systemd user sessions start up or the user sessions will
come up with an incomplete listing of enabled units, etc.

There was a race condition where nothing prevented
`systemd-user-sessions.service` from starting ahead of the systemd user
directory's initialization completing.  This commit makes
`home-manager-<username>` finishes _before_ we start
`systemd-user-sessions.service` to avoid such race condition.

This issue was probably not all that noticeable in most cases, but when using a
non-persistent root config (i.e. tmp on / or
https://grahamc.com/blog/erase-your-darlings) the race condition triggering
causes all kinds of issues on each reboot.

nixos/default.nix

@@ -129,6 +129,7 @@ in {
         wantedBy = [ "multi-user.target" ];
         wants = [ "nix-daemon.socket" ];
         after = [ "nix-daemon.socket" ];
+        before = [ "systemd-user-sessions.service" ];
 
         environment = serviceEnvironment;