scripts: Use /usr/ucb/ps on Solaris instead of the default non-GNU compatible version

Building documentation on Solaris throws this error:

    bin/zotonic generate-edoc
    usage: ps [ -aAdeflcjLPyZ ] [ -o format ] [ -t termlist ]
            [ -u userlist ] [ -U userlist ] [ -G grouplist ]
            [ -p proclist ] [ -g pgrplist ] [ -s sidlist ] [ -z zonelist ]
      'format' is one or more of:
            user ruser group rgroup uid ruid gid rgid pid ppid pgid sid taskid ctid
            pri opri pcpu pmem vsz rss osz nice class time etime stime zone zoneid
            f s c lwp nlwp psr tty addr wchan fname comm args projid project pset

This is due to non-standard `ps` command. The GNU-compatible `ps` command is located in a different directory, hence the need to use the correct version. Tested on Solaris and FreeBSD.