sway: Make it possible to use exec in extraSessionCommands

This is useful for use-cases like this (logging to journald):
```
programs.sway.extraSessionCommands = ''
  if [ -z "$_SWAY_DID_SYSTEMD_CAT" ]; then
    export _SWAY_DID_SYSTEMD_CAT=1
    exec ${config.systemd.package}/bin/systemd-cat -t sway "$0" "$@"
  fi
  # ... (potentially also another exec)
'';
```

Without this change the rest of the extraSessionCommands won't be
executed after the exec since the whole extraSessionCommands block would
be skipped during the re-execution (_SWAY_WRAPPER_ALREADY_EXECUTED is
already set).