josh-proxy.rs: bind dualstacked

[flokli]

Binding on 0.0.0.0 will cause josh to only listen on IPv4, while specifying [::] will bind on both IPv4 and IPv6.

With this change:

❯ ss -ltpn | grep 8000
LISTEN 0      128                *:8000            *:*    users:(("josh-proxy",pid=1894190,fd=10))

[flokli]

Thanks!