Fixes #5594: use windows compatible command for executor daemon

i do not know, as stated earlier, CFEngine best practice is to use for windows the full path, as showed in https://github.com/cfengine/masterfiles/blob/master/controls/cf_execd.cf#L34

the change i'm introducing is replacing && (that doesn't work on Windows) by & and using the syntax that is maintained by CFEngine, and assured to work in current and future release