fix(xo-server/api): log instead of rejecting non-strict schemas

Fixes https://xcp-ng.org/forum/topic/4439/plugin-transport-email-v0-6-0-broken

packages/xo-server/src/xo-mixins/plugins.js

@@ -13,6 +13,7 @@ const log = createLogger('xo:xo-mixins:plugins')
 export default class {
   constructor(xo) {
     this._ajv = new Ajv({
+      strict: 'log',
       useDefaults: true,
     })
     this._plugins = { __proto__: null }