Add boolean input for disabling ipv6

To enable matrix strategy for ipv6 and ipv4 envs, it could be handy to have an optional input arg to disable ipv6

```
grep -qxF 'net.ipv6.conf.all.disable_ipv6=1' /etc/sysctl.conf || echo 'net.ipv6.conf.all.disable_ipv6=1' >> /etc/sysctl.conf
grep -qxF 'net.ipv6.conf.default.disable_ipv6=1' /etc/sysctl.conf || echo 'net.ipv6.conf.default.disable_ipv6=1' >> /etc/sysctl.conf
grep -qxF 'net.ipv6.conf.lo.disable_ipv6=1' /etc/sysctl.conf || echo 'net.ipv6.conf.lo.disable_ipv6=1' >> /etc/sysctl.conf

sysctl -p
```

The issue names /etc/sysctl.conf and sysctl -p as the operations to support. Start by tracing how this GitHub Action defines and executes inputs, then verify that the optional input applies the three listed settings and runs sysctl -p; done means IPv6 can be disabled selectively for matrix jobs without changing default behavior.

Written by the indexing model from the issue text.