cni: add loopback to linux bridge (#13428)

CNI changed how to bring up the interface in v0.2.0.
Support was moved to a new loopback plugin.

https://github.com/containernetworking/cni/pull/121

Fixes #10014

.changelog/13428.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+cni: Fixed a bug where loopback address was not set for all drivers
+```

client/allocrunner/networking_bridge_linux.go

@@ -142,6 +142,9 @@ const nomadCNIConfigTemplate = `{
 	"cniVersion": "0.4.0",
 	"name": "nomad",
 	"plugins": [
+		{
+			"type": "loopback"
+		},
 		{
 			"type": "bridge",
 			"bridge": "%s",