activerecord: CollectionProxy#build can take an array of EachPair

As described above, `CollectionProxy#build` takes an array of attributes
and returns list of instances of the model.

gems/activerecord/6.0/activerecord.rbs

@@ -825,7 +825,8 @@ module ActiveRecord
       #
       #   person.pets.size  # => 5 # size of the collection
       #   person.pets.count # => 0 # count from database
-      def build: (?_EachPair attributes) ?{ () -> untyped } -> untyped
+      def build: (Array[_EachPair] attributes) ?{ () -> untyped } -> Array[untyped]
+               | (?_EachPair attributes) ?{ () -> untyped } -> untyped
 
       alias new build