Fix changing env variable type

With the following:

	koyeb service update myapp/svc --env MYENV=value
	koyeb service update myapp/svc --env MYENV=@xxx

We used to generate a bad request because this is changing the type of
the environment variable MYENV to "plain", then "secret". This is now
fixed.

pkg/koyeb/flags_list/env.go

@@ -78,7 +78,9 @@ func (f *FlagEnv) UpdateItem(env *koyeb.DeploymentEnv) {
 	env.Key = koyeb.PtrString(f.key)
 	if f.isSecret {
 		env.Secret = koyeb.PtrString(f.value)
+		env.Value = nil
 	} else {
+		env.Secret = nil
 		env.Value = koyeb.PtrString(f.value)
 	}
 }