Dynamically get network interface name in domainjoin_unix_script.go

As part of the domain join script for Unix, there is a line where we get the interface IP address, but the interface name is hard-coded (`eth0`) as you can see here: https://github.com/aws/amazon-ssm-agent/blob/baf58f0dedc4dc84d719101259cea5ed73c9f842/agent/plugins/domainjoin/domainjoin_unix_script.go#L555

Although this works for `t2` instance types, it doesn't work for `t3`and `t3a` since the network interface name has changed to `ensX`.

I suggest to change this line to dynamically fetch the the IP without specifying the interface name.
There are 2 options to do this:

```bash
# Option 1:
ip_address="$(hostname -I)"

# Option 2:
ip_address="$(ip -o -4 addr show $(ip route list | grep default | grep -E 'dev (\w+)' -o | awk '{print $2}') | awk '{print $4}' | cut -d/ -f1)"
```

This is an important fix as there is currently an issue with this script when running on newer machine types

Start in agent/plugins/domainjoin/domainjoin_unix_script.go around line 555, where the Unix domain-join script reads the interface IP using the hard-coded eth0 name. Compare the two proposed shell approaches and verify the script obtains the active interface IP on both eth0 and ensX systems. Done means the domain-join script no longer depends on a fixed interface name.

Written by the indexing model from the issue text.