[12.x] Support Eloquent builders and relations as subqueries to update queries

[axlon]

This PR adds support for Eloquent\Builder and Relation in Query\Builder::update(), allowing for simpler update queries:

FooModel::where('...')->update([
-    'bar_id' => BarModel::where('...')->toBase()->select('id'),
+    'bar_id' => BarModel::where('...')->select('id'),
]);

If-condition was taken from:

framework/src/Illuminate/Database/Query/Builder.php

Line 421 in bcaa68a

if ($query instanceof self || $query instanceof EloquentBuilder || $query instanceof Relation) {