exclude default bridge network in network prune

[tal66]

Hi, this should fix #1295
(nerdctl network prune does not need to remove the default bridge network)

[manugupt1]

Please sign off with real name.

[fahedouch]

plz use DefaultNetworkDriver const from package main so we avoid deleting bridge for unix and nat for windows
see #1279 (comment)

[fahedouch]

Thanks, but please squash your commits and sign the commit for DCO
https://github.com/apps/dco

(run git commit -a -s --amend, and make sure that the Signed-off-by: NAME <EMAIL> line with your real name is included in the commit message)

[AkihiroSuda]

Thanks