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).
