Control over micro (exact) versions of Xcode used to build

Description of the feature request

The ability for select the micro version of Xcode. For example: using Xcode 11.4 and not Xcode 11.4.1

Use case / for what or how I would use it

The new version of Xcode 11.4.1 has a new version of Swift. Changes in Swift versions can cause a cascade of dependency issues when released. This means requesting and waiting for new sdk versions of frameworks to be built with the latest Swift from our vendors. Because this is a business case, I do not think Bitrise should be forcing us into the specific version of Xcode being used to build our apps.