cmd: fix boolean backend flags - fixes #2402

Before this change, boolean flags such as `--b2-hard-delete` were
failing to be recognised unless they had a parameter.

This bug was introduced as part of the config re-organisation:
f3f48d7d49

cmd/cmd.go

@@ -523,7 +523,7 @@ func AddBackendFlags() {
 				}
 				help = strings.TrimSpace(help)
 				flag := pflag.CommandLine.VarPF(opt, name, string(opt.ShortOpt), help)
-				if _, isBool := opt.Value.(bool); isBool {
+				if _, isBool := opt.Default.(bool); isBool {
 					flag.NoOptDefVal = "true"
 				}
 				// Hide on the command line if requested