Workaround a WSL systemd issue.

WSL systemd doesn't reliably provide a user session; that is, `systemctl --user status` may fail.
This commit provides a workaround based on:
  microsoft/WSL#10205 (comment)

config/wezterm/wezterm.lua

@@ -28,7 +28,15 @@ config.wsl_domains = {
  {
  name = 'WSL:NixOS',
  distribution = 'NixOS',
- default_prog = { '/run/current-system/sw/bin/fish' },
+ default_prog = {
+ '/run/current-system/sw/bin/fish',
+ '-c',
+ -- to get a working user login session
+ 'while not test -S /run/dbus/system_bus_socket; sleep 1; end;' ..
+ 'if test -e /run/user/1000/bus; exec fish; end;' ..
+ 'sudo systemctl restart user@1000.service;' ..
+ 'exec fish;',
+ },
  default_cwd = '~',
  },
 }