improve script binary location - (update-ipsets no longer update ipsets)

[FabioPedretti]

This assume update-ipsets is called from crontab.
On Debian 8 update-ipsets properly update ipsets if they are already available.
On Debian 11 and 12 update-ipsets update the lists, but no longer update ipsets, even if they are already available.
update-ipsets is the same file on both systems.

Note that this is due to newer Debian having different binary locations. On Debian 11 crontab uses this PATH: /usr/bin:/bin, while ipset (called from update-ipsets script) was moved to /usr/sbin/ipset. Adding a proper PATH when running update-ipsets from crontab fixes it:
PATH=/usr/sbin:/usr/bin:/sbin:/bin /usr/sbin/update-ipsets

This may eventually be improved in firehol configuration / build, to use proper path matching the system.