fix: add IPKG_INSTROOT to post install script
This fixes: ERROR:Failed operations: fix-pkglists-hardening-options/postinst: /builds/turris/turris-build/root/usr/lib/opkg/info//fix-pkglists-hardening-options.postinst: 2: exec: /usr/libexec/fix/pkglists-hardening-options: not found