selftests/bpf: S/iptables/iptables-legacy/ in the bpf_nf and xdp_synproxy test

The recent vm image in CI has reported error in selftests that use
the iptables command.  Manu Bretelle has pointed out the difference
in the recent vm image that the iptables is sym-linked to the iptables-nft.
With this knowledge,  I can also reproduce the CI error by manually running
with the 'iptables-nft'.

This patch is to replace the iptables command with iptables-legacy
to unblock the CI tests.

Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: David Vernet <void@manifault.com>
Link: https://lore.kernel.org/bpf/20221012221235.3529719-1-martin.lau@linux.dev