hush: set $n properly for "source" builtin

hush: set $n properly for "source" builtin

function                                             old     new   delta
restore_G_args                                         -      78     +78
save_and_replace_G_args                                -      64     +64
builtin_source                                        72     107     +35
run_list                                            2549    2367    -182
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(add/remove: 2/0 grow/shrink: 1/1 up/down: 177/-182)           Total: -5 bytes