ash,hush: fix $RANDOM in children being repeated

ash,hush: fix $RANDOM in children being repeated

function                                             old     new   delta
next_random                                           46      68     +22
forkshell                                            248     263     +15
expand_vars_to_list                                 2118    2131     +13
run_pipe                                            1775    1782      +7
popstring                                            134     140      +6
builtin_umask                                        123     121      -2
ash_main                                            1356    1336     -20
get_local_var_value                                  125     104     -21
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(add/remove: 0/0 grow/shrink: 5/3 up/down: 63/-43)             Total: 20 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>