libcni: set both GC valid attachment keys

I made an (embarassing) error where SPEC.md and libcni disagreed on the
key for valid attachments for a GC operation. The spec has been
updated, but libcni should set both keys as a transition mechanism.

Signed-off-by: Casey Callendrello <c1@caseyc.net>

libcni/api.go

@@ -817,6 +817,8 @@ func (c *CNIConfig) GCNetworkList(ctx context.Context, list *NetworkConfigList,
 		}
 		if args != nil {
 			inject["cni.dev/valid-attachments"] = args.ValidAttachments
+			// #1101: spec used incorrect variable name
+			inject["cni.dev/attachments"] = args.ValidAttachments
 		}
 
 		for _, plugin := range list.Plugins {