pinger/src/linux.rs: use "ping -6" instead of "ping6"

[ryanmjacobs]

pinger/src/linux.rs: use "ping -6" instead of "ping6" ping6 does not exist on my system (iputils 20250605-1)

gping -6 fails otherwise.

$ gping -6 example.com
Error: Error spawning ping: No such file or directory (os error 2)

Caused by:
    No such file or directory (os error 2)

Note: Probably don't need to modify the BusyBox command. However, busybox ping supports the same -4/-6 flags:

$ busybox ping --help
...
-4,-6           Force IP or IPv6 name resolution