[LXC] CI fails b/c the test script assumes a specific `iptables` chain exists

### Relevant area(s)

Linux

### Brief description of your issue

Occurs on RHEL, Debian, and Ubuntu 26/24.

Job: https://github.com/microsoft/mxc/actions/runs/35276447087/job/105393731292
Line 639 - 668:

```
nsenter -t 10630 -n iptables -D INPUT -j MXCI-CLI-LX-p6xjfbwhacldb2tr failed: iptables v1.8.11 (nf_tables): Chain 'MXCI-CLI-LX-p6xjfbwhacldb2tr' does not exist
Try `iptables -h' or 'iptables --help' for more information.

nsenter -t 10630 -n iptables -F MXCI-CLI-LX-p6xjfbwhacldb2tr failed: iptables: No chain/target/match by that name.

nsenter -t 10630 -n iptables -X MXCI-CLI-LX-p6xjfbwhacldb2tr failed: iptables: No chain/target/match by that name.

nsenter -t 10630 -n ip6tables -D INPUT -j MXCI-CLI-LX-p6xjfbwhacldb2tr failed: ip6tables v1.8.11 (nf_tables): Chain 'MXCI-CLI-LX-p6xjfbwhacldb2tr' does not exist
Try `ip6tables -h' or 'ip6tables --help' for more information.

nsenter -t 10630 -n ip6tables -F MXCI-CLI-LX-p6xjfbwhacldb2tr failed: ip6tables: No chain/target/match by that name.

nsenter -t 10630 -n ip6tables -X MXCI-CLI-LX-p6xjfbwhacldb2tr failed: ip6tables: No chain/target/match by that name.

Executing script inside container...
MXC_LOOPBACK_OK

MXC_NET_BLOCKED

Removing iptables/ip6tables chain: MXC-CLI-LXC-p6xjfbwhacldb2tr
Removing inbound iptables chain: MXCI-CLI-LX-p6xjfbwhacldb2tr
nsenter -t 10630 -n iptables -D INPUT -j MXCI-CLI-LX-p6xjfbwhacldb2tr failed: iptables: Bad rule (does a matching rule exist in that chain?).

nsenter -t 10630 -n ip6tables -D INPUT -j MXCI-CLI-LX-p6xjfbwhacldb2tr failed: ip6tables: Bad rule (does a matching rule exist in that chain?).

Destroying container...
Runner completed in 3284ms
Exit code: 0 (0x00000000)
SKIP: the control could not reach 140.82.114.6:443 under a policy that permits it, so this host has no outbound path; a blocked result in the case under test would prove nothing.
SKIP: LXC Network 0.8 Deny-All Loopback
```

### Steps to reproduce

See above

### Expected behavior

The test script runs on any machine, without requiring a specific chain to preexist.

### Actual behavior

See above