building Joi from spec fails when example object contains type property

#### Support plan

* *is this issue currently blocking your project?* (yes/no): yes
* *is this issue affecting a production system?* (yes/no): yes

#### Context

* *node version*: 12.18.3
* *module version with issue*: 17.1.1
* *last module version without issue*: -
* *environment* (e.g. node, browser, native): node
* *used with* (e.g. hapi application, another framework, standalone, ...): standalone
* *any other relevant information*:

#### What are you trying to achieve or the steps to reproduce?

Trying to build Joi object from JSON specification which contains examples. When example is set to an object which contains property `type` - build process fails with an exception.

```js
// succeeds
Joi.build({
type: 'object',
examples: [ { name: 'some-type' } ],
keys: { name: { type: 'string' } }
});

// fails with TypeError: this.joi[desc.type] is not a function
Joi.build({
type: 'object',
examples: [ { type: 'some-type' } ],
keys: { type: { type: 'string' } }
});
```

Failing example using .describe()
```js
Joi.build(
Joi.object({
type: J.string(),
}).example({
type: 'some-type',
}).describe(),
);
```

#### What was the result you got?

the error was thrown

```
TypeError: this.joi[desc.type] is not a function
at internals.Builder.parse (node_modules/@hapi/joi/lib/manifest.js:263:41)
at internals.Builder.build (node_modules/@hapi/joi/lib/manifest.js:449:25)
at /Users/user/Developer/project/node_modules/@hapi/joi/lib/manifest.js:393:44
at Array.map ()
at internals.Builder.build (node_modules/@hapi/joi/lib/manifest.js:393:25)
at internals.Builder.parse (node_modules/@hapi/joi/lib/manifest.js:374:31)
at Object.exports.build (node_modules/@hapi/joi/lib/manifest.js:242:20)
```

#### What result did you expect?

Joi object should be successfully created regardless of the properties defined in the example object.