coercions during alternative match leads to unexpected results

#### 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*: 14
* *module version with issue*: 17.x
* *last module version without issue*: n/a
* *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?

Coercions occurring during an alternative match do not take precedence over the uncoerced values during the subschema response value merging, resulting in uncoerced properties overwriting coerced ones.

```js
// coercion works as expected when not inside `alternatives`
joi.object({ org_id: joi.number().required() }).unknown().validate({ org_id: '5', foo: 'bar' }, { convert: true })
// === { value: { org_id: 5, foo: 'bar' } }

// coercion fails when inside `alternatives`
joi.alternatives().try(
joi.object({ org_id: joi.number().required() }).unknown(),
joi.object({ foo: joi.string().required() }).unknown()
).match('all').validate({ org_id: '5', foo: 'bar' }, { convert: true })
// === { value: { org_id: '5', foo: 'bar' } }
```

#### What was the result you got?

org_id: '5'

#### What result did you expect?

org_ig: 5